Therefore I can't change it to my (modified) sbus_mmio_map() function
because it would break other non-SPARC platforms, and AIUI there is nothing
in the memory API that allows me to move a subregion to a different
MemoryRegion parent, even if I can get a reference to it with
sysbus_mmio_get_region() after the sysbus_mmio_map() call - or have I
misunderstood something?
Sysbus is used as a generic class for motherboard devices, there is an
assumption that there is no higher level bus. What we need here is a
full blown bus. The translations and mappigs between bus addresses and
motherboard addresses should be done in a Sysbus to SBus bridge
device, just like PCI host bridges do.