spapr_pci: Fix extended config space accesses

spapr_pci: Fix extended config space accesses

The PAPR PHB acts as a legacy PCI bus but it allows PCIe extended
config space accesses anyway (for pseries-2.9 and newer machine
types).

Introduce a specific PCI bus subtype to inform the common PCI code
about that.

Fixes: c2077e2ca0da7
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <155414130834.574858.16502276132110219890.stgit@bahia.lan>
[dwg: Apply fix so we don't rename the default pci bus, breaking everything]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>