virtio-rng: use virtqueue_get_avail_bytes, fix migration
Popping an elem from the vq just to find out its length causes problems
with save/load later on. Use the new virtqueue_get_avail_bytes()
function instead, saves us the complexity in the migration code, as well
as makes the migration endian-safe.
Signed-off-by: Amit Shah <amit.shah@redhat.com> Reviewed-by: Anthony Liguori <aliguori@us.ibm.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>