pseries: Don't expose PCIe extended config space on older machine types

bb9986452 "spapr_pci: Advertise access to PCIe extended config space"
allowed guests to access the extended config space of PCI Express devices
via the PAPR interfaces, even though the paravirtualized bus mostly acts
like plain PCI.

However, that patch enabled access unconditionally, including for existing
machine types, which is an unwise change in behaviour.  This patch limits
the change to pseries-2.9 (and later) machine types.

Suggested-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
