If we are to cope with multiple domains with clashing PCI bus numbers,
we must refrain from creating two directories of the same name in
/proc/bus/pci.  This is one solution to the problem; busses with a
non-zero domain number get it prepended.

Alternative solutions include cowardly refusing to create non-domain-zero
bus directories, refusing to create directories with clashing names, and
sticking our heads in the sand and pretending the problem doesn't exist.