bus: do not unref hotplug handler

Apparently, none of the bus owner give a reference to the hotplug
handler property, do not unref it on bus release.

Furthermore, a bus is allowed to be its own hotplug handler, which can
be seen in qbus_set_bus_hotplug_handler() function. However, in this
case, the reference can't be given to the property, or this will create
a cyclic dependency and the bus will never be free.

Each bus owner should manage the lifecycle of the hotplug handler.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

hw/acpi/piix4.c

@@ -440,6 +440,8 @@ static void piix4_update_bus_hotplug(PCIBus *pci_bus, void *opaque)
 {
     PIIX4PMState *s = opaque;
 
+    /* pci_bus cannot outlive PIIX4PMState, because /machine keeps it alive
+     * and it's not hot-unpluggable */
     qbus_set_hotplug_handler(BUS(pci_bus), DEVICE(s), &error_abort);
 }
 

hw/core/bus.c

@@ -197,7 +197,7 @@ static void qbus_initfn(Object *obj)
                              TYPE_HOTPLUG_HANDLER,
                              (Object **)&bus->hotplug_handler,
                              object_property_allow_set_link,
-                             OBJ_PROP_LINK_UNREF_ON_RELEASE,
+                             0,
                              NULL);
     object_property_add_bool(obj, "realized",
                              bus_get_realized, bus_set_realized, NULL);