virtio_net: pass vi around

Too many places poke at [rs]q->vq->vdev->priv just to get
the vi structure.  Let's just pass the pointer around: seems
cleaner, and might even be faster.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>