conf: virDomainDefMaybeAddController tweak return code
conf: virDomainDefMaybeAddController tweak return code

Previously the function returned either -1 in case of an error or 0 on
success. However, we should also distinguish between a case we
successfully added a controller and a case there wasn't a need to add any
controller

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