core: move unplugged disks while VM is running

Refactor/rewrite of MoveDisksCommand to support the following:
* Offline move of unplugged disks while the VM is running.
* Add error messages when trying to move/migrate a mixture
  of plugged and unplugged disks (on a running VM).

Change-Id: Ie0fbfd95c0086cfe0d792b3071a2d804ff32c18e
Bug-Url: https://bugzilla.redhat.com/987783
Bug-Url: https://bugzilla.redhat.com/957494
Bug-Url: https://bugzilla.redhat.com/957492
Signed-off-by: Daniel Erez <derez@redhat.com>