s390x/css: attach css bridge

Logically, the css bridge should be attached to the machine.

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Tested-by: Bjoern Walk <bwalk@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

hw/s390x/css-bridge.c

@@ -99,6 +99,8 @@ VirtualCssBus *virtual_css_bus_init(void)
 
     /* Create bridge device */
     dev = qdev_create(NULL, TYPE_VIRTUAL_CSS_BRIDGE);
+    object_property_add_child(qdev_get_machine(), TYPE_VIRTUAL_CSS_BRIDGE,
+                              OBJECT(dev), NULL);
     qdev_init_nofail(dev);
 
     /* Create bus on bridge device */