This utility function iterates through all devices looking for any
with a PCI address that has function != 0 (which implies that multiple
functions are in use on that slot), then uses an inner iterator to
find the device that's on function 0 of that same slot and sets the
"multi" in its virDomainDeviceInfo (as long as it hasn't already been
set explicitly by someone who presumably has better information than
we do).

It isn't yet called from anywhere, so will have no functional effect.