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>
