s390x/css: introduce property type for device ids

Let's introduce a CssDevId to handle device ids of the xx.x.xxxx
type used for channel devices. This has some benefits:

- We can use them in virtio-ccw and split the validity checks for
  a channel device id in general from the constraint checking
  within the virtio-ccw scope.
- We can reuse the device id type for future non-virtio channel
  devices.

While we're at it, improve the validity checks and disallow e.g.
trailing characters.

Suggested-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Acked-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>

hw/s390x/virtio-ccw.h

@@ -80,7 +80,7 @@ typedef struct VirtIOCCWDeviceClass {
 struct VirtioCcwDevice {
     DeviceState parent_obj;
     SubchDev *sch;
-    char *bus_id;
+    CssDevId bus_id;
     int revision;
     uint32_t max_rev;
     VirtioBusState bus;