net/9p: Use virtio transpart as the default transport

Make the default 9p experience better by defaulting to virtio transport if present.
These days most of the users are using 9p in a virtualized setup

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>