Location plugin loading code potentially leaks memory

The plugin loading code used for the Maps and Navigation API and the Landmarks API (and, from initial grepping of the source tree, Contacts and Organizer) doesn't unload plugins which are placed in the wrong directory.

In these APIs QPluginLoader::instance() is called inside a qobject_cast. If the cast is unsuccessful nothing is done to call QPluginLoader::unload() for the plugin.

The Feedback, Sensors, Multimedia and Service Framework APIs all appear to make use of QPluginLoader::unload(), so ideas could be gathered from how those APIs deal with the problem.