core:unlock_entity is not locating locked snapshots

Adding handling of
1) -t snapshot flag to display locked snapshots where -q is used

2) -t snapshot to remove a specific snapshot

3) -r if run with -t vm then all vm snapshots are removed as well

Change-Id: I415be6fbe99a97740c9d4e9c55c1ad63bc689531
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=958370
Signed-off-by: Eli Mesika <emesika@redhat.com>