cfg.mk: Use single quotes wherever possible
cfg.mk: Use single quotes wherever possible

Being consistent is nice, especially when it comes to defining our
regular expression, where using single quotes instead of double
quotes allows us to leave out a few backslashes.

Changing this required altering a few error messages.

The only remaining use of double quotes is one where they are
actually required for the check to work.

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