Fix xml renderer

[sveneberth]

The xml renderer was always left behind in the past and sometimes has errors (such as this undefined symbol render)

Maybe in the future the html renderer can also inherit from the xml renderer and some parts can be moved.

[phorward]

Maybe in the future the html renderer can also inherit from the xml renderer and some parts can be moved.

I see this as part of #1071 and would generally advise to drop the renderer entirely for now, and re-implement it later on demand using a master renderer constructing JSON-serializable objects which simply is dumped to the specific target format, like JSON/MsgPack or even XML.