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