9p: add remove function to trans_virtio

Request from rusty:
Just cleaning up patches for 2.6.25 merge, and noticed that
net/9p/trans_virtio.c doesn't have a remove function.  This will crash when
removing the module (console doesn't have one because it can't really be
removed).

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