net/9p: Add device name details on error

If we use wrong device name 9p mount fails with error

"9pnet_virtio: no channels available"

Improve the error output as below

"9pnet_virtio: no channels available for device /dev/root"

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