core: Snapshot failure should not be retried.

When snapshot VDS command throws any exception, engine should end the
command with an appropriate audit log which indicates that a restart is required,
instead of retrying the VDS command every 10 seconds and flooding the event log.

Change-Id: I8f56b025e0c6bbd9c9992ee65007a19c97658790
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Bug-Url: https://bugzilla.redhat.com/923366