virtio-rng: cleanup: use QOM casts.

virtio-rng: cleanup: use QOM casts.

As the virtio-rng-pci, virtio-rng-s390 and virtio-rng-ccw are
switched to the new API, we can use QOM casts.

Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Tested-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: Amit Shah <amit.shah@redhat.com>
Message-id: 1366790881-3026-9-git-send-email-fred.konrad@greensocs.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>