Fix crash-on-exit with some plugin systems (e.g. sensors)

Don't assume that all entries in the QLibrary global data refer to
libraries have have been loaded. There are three (not two) ways of
getting entries there:

 1) creating a QLibrary
 2) using the static QLibrary::resolve
 3) via plugins

The unload code was meant to handle the first two cases only: libraries
are still loaded at the end of the execution if the static methods were
called or if QLibrary objects were leaked. It didn't handle the case of
plugins being found by the directory scanner in QFactoryLoader but never
loaded.

Note it's possible that this assertion also happened with leaked
QLibrary.

Change-Id: Idcd7a551f96d8fe500cbca682f8014f5122b7584
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>