libvirt is happy with redefinition of an existing nwfilter

Change-Id: Id253c66df8296613b6a2bc3fc058ef2754702964
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20788
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Moti Asayag <masayag@redhat.com>