qdev: pass an Object * to qbus_set_hotplug_handler()

Certain devices types, like memory/CPU, are now being handled using a
hotplug interface provided by a top-level MachineClass. Hotpluggable
host bridges are another such device where it makes sense to use a
machine-level hotplug handler. However, unlike those devices,
host-bridges have a parent bus (the main system bus), and devices with
a parent bus use a different mechanism for registering their hotplug
handlers: qbus_set_hotplug_handler(). This interface currently expects
a handler to be a subclass of DeviceClass, but this is not the case
for MachineClass, which derives directly from ObjectClass.

Internally, the interface only requires an ObjectClass, so expose that
in qbus_set_hotplug_handler().

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <154999589921.690774.3640149277362188566.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

hw/s390x/css-bridge.c

@@ -108,7 +108,7 @@ VirtualCssBus *virtual_css_bus_init(void)
     cbus = VIRTUAL_CSS_BUS(bus);
 
     /* Enable hotplugging */
-    qbus_set_hotplug_handler(bus, dev, &error_abort);
+    qbus_set_hotplug_handler(bus, OBJECT(dev), &error_abort);
 
     css_register_io_adapters(CSS_IO_ADAPTER_VIRTIO, true, false,
                              0, &error_abort);