net/9p: remove virtio default hack and set appropriate bits instead

A few releases back a patch made virtio the default transport, however
it was done in a way which side-stepped the mechanism put in place to
allow for this selection.  This patch cleans that up while maintaining
virtio as the default transport.

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

net/9p/trans_virtio.c

@@ -698,7 +698,7 @@ static struct p9_trans_module p9_virtio_trans = {
 	 * page in zero copy.
 	 */
 	.maxsize = PAGE_SIZE * (VIRTQUEUE_NUM - 3),
-	.def = 0,
+	.def = 1,
 	.owner = THIS_MODULE,
 };