vdsm: disable virtio-rng on rhel < 6.6

Virtio-rng support was added in libvirt 1.0.3, and is missing in
EL 6.5 (It is supported since libvirt version 0.10.2-31 only).
The support is indicated by not sending the rngSources in hostCapabilities.

Change-Id: I3493fec50d5d0e055a09f3393c86ff76f2a1ce5a
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28838
Tested-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>