conf: introduce virDomainControllerDriverFormat
conf: introduce virDomainControllerDriverFormat

Split out formatting the <driver> subelement of <controller>
to make adding new options easier.

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