init QtModuleInfo::isPlugin properly

Change-Id: I8e28983e7d74d3196ea1a0c89a7d9a52949b44fe
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
fix setup-qt for modules without libs

The convenience module Qt.test got libraries assigned that do not exist.

Change-Id: I9fa5d1d67ce9f540c0d0f70ae3d0921c37dad7b3
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>