qdev: Add a bus child add/remove handler

Add a way for a bus to detect when something is added or
removed from it.  This way the I2C mux can know when to
add itself to the master bus.

Signed-off-by: Corey Minyard <cminyard@mvista.com>

hw/core/qdev.c

@@ -757,6 +757,10 @@ static void device_set_realized(Object *obj, bool value, Error **errp)
             }
         }
 
+        if (dev->parent_bus && dev->parent_bus->child_added) {
+            dev->parent_bus->child_added(dev->parent_bus, dev);
+        }
+
         if (dc->realize) {
             dc->realize(dev, &local_err);
             if (local_err != NULL) {
@@ -849,6 +853,9 @@ static void device_set_realized(Object *obj, bool value, Error **errp)
         if (dc->unrealize) {
             dc->unrealize(dev);
         }
+        if (dev->parent_bus && dev->parent_bus->child_removed) {
+                dev->parent_bus->child_removed(dev->parent_bus, dev);
+        }
         dev->pending_deleted_event = true;
         DEVICE_LISTENER_CALL(unrealize, Reverse, dev);
     }

include/hw/qdev-core.h

@@ -274,6 +274,8 @@ struct BusState {
     QTAILQ_HEAD(, BusChild) children;
     QLIST_ENTRY(BusState) sibling;
     ResettableState reset;
+    void (*child_added)(BusState *bus, DeviceState *child);
+    void (*child_removed)(BusState *bus, DeviceState *child);
 };
 
 /**