Add a bus method to fetch the VM domain for the given device/bus.

* Add a bus_if.m method - get_domain() - returning the VM domain or
  ENOENT if the device isn't in a VM domain;
* Add bus methods to print out the domain of the device if appropriate;
* Add code in srat.c to save the PXM -> VM domain mapping that's done and
  expose a function to translate VM domain -> PXM;
* Add ACPI and ACPI PCI methods to check if the bus has a _PXM attribute
  and if so map it to the VM domain;
* (.. yes, this works recursively.)
* Have the pci bus glue print out the device VM domain if present.

Note: this is just the plumbing to start enumerating information -
it doesn't at all modify behaviour.

Differential Revision:	D906
Reviewed by:	jhb
Sponsored by:	Norse Corp
Missing from previous commit - keep the VM domain -> PXM mapping
array and use it to map PXM -> VM domain when needed.

Differential Revision:	D906
Reviewed by:	jhb