virtio-ccw: wire up ->bus_name callback

Return the bus id of the ccw proxy device. This makes 'ethtool -i'
show a more useful value than 'virtio' in the bus-info field.

Acked-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>