utils: add unpersist for symmetry

Change-Id: I61fffa6d65a655fa3cbe4342cc4ac7a0bf315129
Signed-off-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34074
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>