Allow drag-and-drop of disks between controllers

[Jim Klimov]

VirtualBox GUI allows to define several disk controllers for a VM (such as SATA, IDE and SCSI controllers). While testing different boot modes (bootloader development) it is needed to switch a single HDD image between controllers, however it is not convenient onw, because a disk image must be removed from the old controller and added to new controller with several mouse clicks and a file selection window. This RFE suggests to allow drag-and-drop of already-attached virtual storage devices between available VM storage controllers, easing this operation to one click-and-drag.