pci: Adjust PCI config limit based on bus topology

A conventional PCI bus does not support config space accesses above
the standard 256 byte configuration space.  PCIe-to-PCI bridges are
not permitted to forward transactions if the extended register address
field is non-zero and must handle it as an unsupported request (PCIe
bridge spec rev 1.0, 4.1.3, 4.1.4).  Therefore, we should not support
extended config space if there is a conventional bus anywhere on the
path to a device.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>