Only return two values in udevGetStringSysfsAttr
Only return two values in udevGetStringSysfsAttr

The callers only care for an error, and a missing attribute
is simply NULL.

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