Enable VM migration to old vdsm's.

Old vdsm's can't run VM from /var/run/vdsm/storage and rely on
/rhev/data-center hierarchy.

Change-Id: Ia778ad743a11b4c1d212857d8f25c81eb4c0defe
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1011608
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19825
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>