BUG-1714: rework XmlDocumentUtils flow

This reworks the flow such that we never override the result of a
codec's transformation. As a drive-by, we reuse the QName exposed in
SchemaContext to reuse the NETCONF QNameModule.

Change-Id: Id4e1c55fc403167436bbf7836b31e0de7f0d6355
Signed-off-by: Robert Varga <rovarga@cisco.com>