WINDING DEVICE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique of a winding device adapted to wind a spun yarn to form a package. 2. Description of the Related Art Conventionally, there is known a winding device adapted to wind a spun yarn unwound from a yarn supplying bobbin to form a package (see e.g., Japanese Unexamined Patent Publication No. 2009-286608). The winding device includes a cradle adapted to rotatably support a package, and a roller adapted to rotate while making contact with the package to assist the winding of the spun yarn. The winding device also includes a lift mechanism adapted to swing t~he cradle to make the package and the roller into contact with each other or to separate the package and the roller away from each other (see e.g., Japanese Unexamined Patent Publication No. 2010-132:09). When power failure occurs, the winding device cuts the spun yarn and separates the package and the roller from each other. This prevents the spun yarn having a defective portion from being wound into the package and also prevents degradation in Lhe quality of the spun yarn already wound into the package. The reasons for separating the package and the roller from each other will be specifically described below. When power failure occurs, the package and the roller rotate by inertia as the power source is lost. Since the inertia moments of the package and the roller are different, a difference is generated between the peripheral speeds thereof. A.s described above, since the spun yarn is cut when the power failure occurs, among the spun yarn already wound into the package, the spun yarn wound around the outer periphery is repeatedly rubbed by the rotation of the roller. As a result, degradation in quality such as hairiness of the spun yarn 2 may be caused. Thus, when the power failure occurs, the winding device separates the package and the roller from each other to prevent the quality of the spun yarn from degrading. There exists a winding device having a configuration in which the package rotates corresponding to the rotation of the rotating roller (see e.g., Japanese Unexamined Patent Publication No. 2011-105460) In this case, the above roller serves as a "traverse drum" adapted to rotate the package while traversing the spun yarn by a guiding groove provided on the surface thereof. In the winding device having such a configuration, the inertia moment of the roller is large, and hence regenerative electric power obtained from the rotation of the roller can be used as the power source of the lift mechanism. That is, if the inertia moment of the roller is large, the regenerative electric power can be stably obtained and the current flow to an electromagnetic valve and the like can be continued. Therefore, the winding device having such a configuration can maintain the separated state after separating the package and the roller from each other. There also exists a winding device having a configuration in which the roller rotates corresponding to the rotation of the rotating package (see e.g., Japanese Unexamined Patent Publication No. 2010-42904). In this case, the above roller serves as a "touch roller" adapted to adjust the shape of the package by pressing the outer peripheral surface of the package. In the winding device having such a configuration, since the inertia moment changes by the outer diameter of the package and the like, it is difficult to use the regenerative electric power obtained from the rotation of the packaqe a.~ the power source of the lift mechanism. Thal is, if the inertia moment of the package is small such as in a state in which the spun yarn is hardly wound, the regenerative electric power cannot be stably obtained, and the current flow to the electromagnetic valve and the like cannot be continued. Therefore, the winding device having such a configuration has a problem in that the separated state cannot be maintained even if the package 3 and the roller are separated from each other, and the package and the roller are made into contact with each other again. In other words, the spun yarn wound into the package is rubbed by the rotation of the roller, thus degrading the quality of the spun yarn. Even in the winding device having a configuration in which the package rotates corresponding to the rotation of the rotating roller, a similar problem occurs when the inertia moment of the roller is small. That is, if the inertia moment of the roller is small, the regenerative electric power cannot be stably obtained, and the current flow to the electromagnetic valve and the like cannot be continued. In a configuration (configuration using the normal open type electromagnetic valve) in which the package and the roller are separated by an urging force of a spring when the power failure occurs and the current flow to the electromagnetic valve is stopped, the electric power for making the package into contact with the roller is constantly consumed in the situation where the power failure has not occurred (winding operation state), and thus the load of Lhe running cost is large. BRIEF SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object thereof is to provide a winding device capable of maintaining a state in ~