spec: vdsm-hook-sriov on el6 should not require non-existing rpm

As reported by a dead horse, el6 does not carry
libvirt-daemon-driver-nodedev so the said hook should not require this
subrpm. Since it the rpm hook is meaningful only when vdsm is installed,
and el6's vdsm requires al libvirt, we need no special requirement
there.

Change-Id: I25bfec65fd017a048102d6321335c747a17b5dd4
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17554
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dead Horse <deadhorseconsulting@gmail.com>
Tested-by: Dead Horse <deadhorseconsulting@gmail.com>