s390x/css: Unplug handler of virtual css bridge

s390x/css: Unplug handler of virtual css bridge

The previous patch moved virtual css bridge and bus out from
virtio-ccw, but kept the direct reference of virtio-ccw specific
unplug function inside css-bridge.c.

To make the virtual css bus and bridge useful for non-virtio devices,
this introduces a common unplug function pointer "unplug" to call
specific virtio-ccw unplug parts. Thus, the tight coupling to
virtio-ccw can be removed.

This unplug pointer is a member of CCWDeviceClass, which is introduced
as an abstract device layer called "ccw-device". This layer is between
DeviceState and specific devices which are plugged in virtual css bus,
like virtio-ccw device. The specific unplug handlers should be assigned
to "unplug" during initialization.

Signed-off-by: Jing Liu <liujbjl@linux.vnet.ibm.com>
Reviewed-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Reviewed-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>