filestorage: Enable the Rename action

Enable the 'Rename' function, which practically moves a file from a given path
to a new one. Also, add the necessary checks to ensure that the new path is
acceptable.
cmdlib: Rename file disk in attach/detach

When detaching a file disk from an instance, move it out of the instance's
directory. When attaching a file disk to an instance, move the disk to that
instance's directory. In both cases, update the logical_id to store the path
where the disk will be stored.
tests.filestorage: Test if rename works properly

Test if the Move operation, which is called by 'Rename', of FileDeviceHelper
works as intended.
tests.cmdlib: Test detach/attach for file disks

Check if the detach/attach operation works as intended for file-based disks. We
check this by looking if the logical_id is changed properly after each action.