proc: Work around broken /proc/bus/pci/ on some architectures.

proc: Work around broken /proc/bus/pci/ on some architectures.

Several architectures (e.g., sparc64) use domains in proc directory
names, which is inconsistent, because /proc/bus/pci/devices is still
unable to represent domains.

Work around it, so that the proc back-end works at least on single-domain
machines.

Suggested by Matthew Wilcox.