[xml] xmlList deallocator callback

["Tony Hanratty" <thanratty blueyonder co uk>]

If you supply a deallocator to xmlListCreate, this is called with an
xmlLinkPtr as a parameter whenever a node is deleted.
But since the xmlLink structure is internal to the library, there's no way
to integrate deleting your node data with the node delete.
Is this by design?

Regards,
Tony.