engine: Deleting all snapshot disks during a vdsm restart leaves disks locked

Restarting vdsm when deleting snapshot volumes
leaves some volumes in locked status.

Change-Id: I58b354541821845f506f9848782763f3c2526f9c
Bug-Url: https://bugzilla.redhat.com/1157222
Signed-off-by: Ravi Nori <rnori@redhat.com>