qdev: get_child_bus(): Use QOM lookup if available

qbus_realize() adds busses as a QOM child of the device in addition to
adding it to the qdev bus list. Change get_child_bus() to use the QOM
child if it is available. This takes priority over the bus-list, but
the child object is checked for type correctness.

This prepares support for aliasing of buses. The use case is SoCs,
where a SoC container needs to present buses to the board level, but
the buses are implemented by controller IP we already model as self
contained qbus-containing devices.

Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Acked-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/core/qdev.c

@@ -580,6 +580,12 @@ void qdev_pass_gpios(DeviceState *dev, DeviceState *container,
 BusState *qdev_get_child_bus(DeviceState *dev, const char *name)
 {
     BusState *bus;
+    Object *child = object_resolve_path_component(OBJECT(dev), name);
+
+    bus = (BusState *)object_dynamic_cast(child, TYPE_BUS);
+    if (bus) {
+        return bus;
+    }
 
     QLIST_FOREACH(bus, &dev->child_bus, sibling) {
         if (strcmp(name, bus->name) == 0) {