core: lsm - unlock all image snapshots on snapshot failure

When failing to create live snapshot, all the relevant disks snapshots
should be unlocked.

Change-Id: Ia583b6ba2f8581865fe5fa1fabcd3feb755bce54
Related To: https://bugzilla.redhat.com/show_bug.cgi?id=1028768
Signed-off-by: Liron Aravot <laravot@redhat.com>