core:sorting of VMs changes during refresh

Adding a default secondary sort that insures that if another sort is
used all entities are sorted with entity name as a secondary field.
In case no sort is used explicitly, the entity name is used as before

Change-Id: If82b26f8b648abbce3a8337eb0ccb1212a268116
Signed-off-by: Eli Mesika <emesika@redhat.com>