virpci: fix build on non-Linux
virpci: fix build on non-Linux

virPCIGetDeviceAddressFromSysfsLink() should return
virPCIDeviceAddressPtr, so return NULL in the stub instead of "-1".

libvirt/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,datto/libvirt,crobinso/libvirt,zippy2/libvirt,libvirt/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,jardasgit/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,crobinso/libvirt,jardasgit/libvirt,crobinso/libvirt,datto/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,crobinso/libvirt,olafhering/libvirt,nertpinx/libvirt,eskultety/libvirt,nertpinx/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,jardasgit/libvirt,jardasgit/libvirt,libvirt/libvirt,zippy2/libvirt,olafhering/libvirt,eskultety/libvirt,andreabolognani/libvirt,datto/libvirt,eskultety/libvirt,VenkatDatta/libvirt,eskultety/libvirt,datto/libvirt,andreabolognani/libvirt,zippy2/libvirt,libvirt/libvirt,andreabolognani/libvirt,datto/libvirt,olafhering/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,eskultety/libvirt,olafhering/libvirt,zippy2/libvirt,fabianfreyer/libvirt