core: clear memory state if the snapshot is not live

This patch solves a bug that was caused by not clearing the memory state
which is stored a snapshot, when the live part of the create snapshot
command is not executed.

Change-Id: Ie34a0c7121f5c407648dcee5d6b9d719ede1306b
Bug-Url: https://bugzilla.redhat.com/985739
Signed-off-by: Arik Hadas <ahadas@redhat.com>