Changed plugin loader to try unloading plugin if unused

Deleting the root component can delete plugins already
used elsewhere in the application or library.

Change-Id: Ibd0afb61cf0e118eb13f9782252c1fec554ea901
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>

src/multimedia/qmediapluginloader.cpp

@@ -225,7 +225,7 @@ void QMediaPluginLoader::load()
 #endif
             }
 
-            delete o;
+            loader.unload();
         }
     }
 }