libqos: Limit spapr-pci to 32-bit MMIO for now

Currently the functions in pci-spapr.c (like pci-pc.c on which it's based)
don't distinguish between 32-bit and 64-bit PCI MMIO.  At the moment, the
qemu side implementation is a bit weird and has a single MMIO window
straddling 32-bit and 64-bit regions, but we're likely to change that in
future.

In any case, pci-pc.c - and therefore the testcases using PCI - only handle
32-bit MMIOs for now.  For spapr despite whatever changes might happen with
the MMIO windows, the 32-bit window is likely to remain at 2..4 GiB in PCI
space.

So, explicitly limit pci-spapr.c to 32-bit MMIOs for now, we can add 64-bit
MMIO support back in when and if we need it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
