(V2) Map hot-plugged BAR into SPAPR processor host bus

When a device is hot-plugged into the PAPR PCI PHB, find an unmapped
memory sub-region within the geusts memory or io address space. If
successful, cause a memory subregion to be mapped into guest memory by
writing the appropriate BARs in the device's config space.

Signed-off-by: Mike Day <ncmike@ncultra.org>