PCI: properly abstract out per-architecture extensions to struct pci_dev

PCI: properly abstract out per-architecture extensions to struct pci_dev

x86's used_vectors member was both misplaced (in struct pci_dev_info,
which acts as a hypercall input data passing container only) and
improperly abstracted (requiring a CONFIG_X86 conditional in a generic
header).

The adjustment requires hiding several more lines in IA64's pci.h, but
as a benefit this allows removing one of the "null" headers.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>