core: TryBackToAllSnapshots - avoid attempt to remove illegal disks

In change I8ee0e807 previewing a snapshot of vm with illegal disks was
blocked, so we will never actually remove any illegal disks as if there
were some - the action would have failed in the CDA phase.

Related-To: https://bugzilla.redhat.com/949624
Change-Id: Ie59bb1de3e9431998c9aa5269e3b65de8165dee9
Signed-off-by: Liron Aravot <laravot@redhat.com>
Signed-off-by: Allon Mureinik <amureini@redhat.com>