xen: create a new PCI bus override

When running as a Xen PVH Dom0 we need to add custom buses that override
some of the functionality present in the ACPI PCI Bus and the PCI Bus. We
currently override the ACPI PCI Bus, but not the PCI Bus, so add a new
override for the PCI Bus and share the generic functions between them.

Reported by: David P. Discher <dpd@dpdtech.com>
Sponsored by: Citrix Systems R&D

conf/files.amd64:
 - Add the new files.

x86/xen/xen_pci_bus.c:
 - Generic file that contains the PCI overrides so they can be used by the
   several PCI specific buses.

xen/xen_pci.h:
 - Prototypes for the generic overried functions.

dev/xen/pci/xen_pci.c:
 - Xen specific override for the PCI bus.

dev/xen/pci/xen_acpi_pci.c:
 - Xen specific override for the ACPI PCI bus.