pxb: Restrict to x86

The PCI Expander Bridge (PXB) device is essentially a hack to allow
different PCIe devices to be assigned to different NUMA nodes on x86.  Each
PXB is sort-of a separate PCI host bridge, except that its config space
is shared with the config space of the main PCI host bridge, rather than
being independent.

This is only necessary if the platform doesn't (easily) allow truly
independent PCI host bridges.  AFAIK that's just x86.

This patch makes it possible to configure PXB out of the build, and adjusts
the default configs so it's only included on x86 targets.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Michael S. Tsirkin <mst@redhat.com>