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.
-u and -U are supposed to affect existing files too.

Reviewed by:	marcel
Add llvm patch corresponding to r275633.
Add more tests for pw(8) useradd.

PR:		195832
Submitted by:	Robert O'Neil <oneil.rs@gmail.com>
Approved by:	will
Break out the tests into a file per command and clean up some long lines

Approved by:	will
Add some tests for user modification. [1]
Fix a missing test in the Makefile from my previous commit.

PR:		195834 [1]
Submitted by:	Robert O'Neil <oneil.rs@gmail.com>
Approved by:	will