x86 PCI domain support

ACPI PCI support stopped short of supporting multiple PCI domains,
which is something I need in order to support a current machine
configuration, and something many will soon need, to support upcoming
systems.

This is a minimal, untested implementation.  But it should work,
provided your PCI op hooks (direct, BIOS, mmconfig) support PCI domains
(mmconfig).

Patches:

1) Fix: don't hardcode PCI domain to zero
2) Introduce struct pci_sysdata
3) Add PCI domain to struct pci_sysdata