s390x/css: Add a callback for when subchannel gets disabled

We need a possibility to run code when a subchannel gets disabled.
This patch adds the necessary infrastructure.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>