core: block MoveDisks on snapshot preview

The moveImage command in VDSM (at the moment of this writing) cannot
move disks of VMs that are in preview mode. An additional validation
has been added to MoveDisksCommand to prevent such action for the time
being.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=865021
Change-Id: Ib3b93d08eaadb230fe2cc985a9ba57efb5dd3abb
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>