bugfix: fixed libxml2 compilation error (for versions >= 2.9.0)

More info:
https://git.gnome.org/browse/libxml2/commit/?id=e258adecd0
http://xmlsoft.org/html/libxml-tree.html#LIBXML2_NEW_BUFFER

Since libxml2 2.9.0 one must access xmlParserInputBuffer and
xmlOutputBuffer structures through functions xmlOutputBufferGetContent()
and xmlOutputBufferGetSize(). The old way (directly accessing structure
fields) no longer works.

It affects two files in 'paramd'.