virtio-spec: document virtio-9p

Add basic documentation for virtio-9p. I can expand more on device operation,
but I don't think there's anything significant enough for the spec to be
mentioned there. Please let me know if I'm wrong.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>