Symmetrize *StorageDomain.*activateVolumes().

Related to BZ#960952, BZ#769502.

Change-Id: Id182158d54df75c712222061b005be97fc933529
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/16425
Reviewed-by: Ayal Baron <abaron@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>