spec: bump libvirt version for el6

resolves libvirt stability issues (e.g. BZ #922063)

Change-Id: Icbd9c1a6885b9b7ce9ea58b8be2915bf3bc4f1c6
Signed-off-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14120
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>