s390x/css: consolidate the devno property for ccw devices

'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>
diff --git a/hw/s390x/ccw-device.h b/hw/s390x/ccw-device.h
index 48700fe..89c8e5d 100644
--- a/hw/s390x/ccw-device.h
+++ b/hw/s390x/ccw-device.h
@@ -20,7 +20,7 @@
     SubchDev *sch;
     /* <cssid>.<ssid>.<device number> */
     /* The user-set busid of the virtual ccw device. */
-    CssDevId bus_id;
+    CssDevId devno;
     /* The actual busid of the virtual ccw device. */
     CssDevId dev_id;
     /* The actual busid of the virtual subchannel. */