Add a sysbus_mmio_get_region() which allows users of sysbus
devices to turn a (SysBusDevice*, mmioidx) tuple into a
MemoryRegion*. This enables some useful simplifications of
devices which pass through another device's mmio region
(either directly or by implementing some kind of memory
controller device).
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>