pci: Fix virPCIGetPhysicalFunction()'s callers

pci: Fix virPCIGetPhysicalFunction()'s callers

Commit c8b1a83605e4 changed the function, making it
impossible for callers to be able to tell whether a
non-negative return value means "physical function
address found and parsed correctly" or "couldn't find
corresponding physical function".

The important difference between the two being that,
in the latter case, the returned pointer is NULL and
should never, ever be dereferenced.

In order to cope with these changes, the callers
have to be updated.