Use a separate buffer for <hub> subelements
Use a separate buffer for <hub> subelements

Switch virDomainHubDefFormat to use a separate buffer for subelements.

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