xen/pci: Try harder to get PXM information for Xen

If the device being added to Xen is not contained in the ACPI table,
walk the PCI device tree to find a parent that is contained in the ACPI
table before finding the PXM information from this device.

Previously, it would try to get a handle for the device, then the
device's bridge, then the physfn.  This changes the order so that it
tries to get a handle for the device, then the physfn, the walks up the
PCI device tree.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>