'devno' should rather be a property of the ccw device, instead of a
property of a specific virtio-ccw device. Let's consolidate it.
While we are at here, also rename CcwDevice.bus_id to CcwDevice.devno to
make things clearer.
Signed-off-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>