Check supervdsm execution return code

Change-Id: I1b15efaabd2e45eb2f752d9f2ad73400033f2983
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14428
Reviewed-by: Dan Kenigsberg <danken@redhat.com>