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 <[email protected]>
Reviewed-by: Sascha Silbe <[email protected]>
Reviewed-by: Dong Jia Shi <[email protected]>
Reviewed-by: Yi Min Zhao <[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>