Fix a crash in QFactoryLoader

Fix a crash in QFactoryLoader

The change in plugin loading has meant that different plugins in the
same plugin folder will not be handled properly when loaded with
different instances of QFactoryLoader.
A solution is to only unload compatability plugins from
QFactoryLoader::update() since they are the only plugins that are
actually loaded in that method.
This auto test shows the error on the current version of QFactoryLoader
and passes with the fix described above.

Change-Id: I12001525d51bb631d6742c5965357598322f247c
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>