conf: Use virXMLFormatElement in virDomainControllerDefFormat
conf: Use virXMLFormatElement in virDomainControllerDefFormat

Refactor the function to use the XML formatting aid and use automatic
cleaning to simplify the control flow.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

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