xml-processing: Small documentation corrections

XbelReader and XbelWriter do not subclass QXmlStreamReader and
QXmlStreamWriter anymore, but use aggregation instead.

Change-Id: Idbe43e9bdbc47dc73b1984ce17ae4d18e9be5554
Reviewed-by: Thorbjørn Lindeijer <bjorn@lindeijer.nl>
Make the CMake tests run sequentially, not in parallel.

Otherwise the output is too intertwined.

Change-Id: I6729727b3afcdcbec58e3fa560587dd1fa08f38e
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>