validate_dbus_object can cause more than one class in the cpo cache

When using validate_dbus_object to override the custom proxy object match, the real QML classname can get to the cache and autopilot will fail saying that there are two possible classes that match the query.

For example, on system settings we have many ItemPage qml objects. Each one has different functions and children, so I made different python classes overriding the validate_dbus_object, like here: http://bazaar.launchpad.net/~system-settings-touch/ubuntu-system-settings/trunk/view/head:/tests/autopilot/ubuntu_system_settings/__init__.py#L192

The problem is that if you select_single('ItemPage') at some point, the ItemPage will be added to the cache and it will make any try to select the other pages objects like AboutPage to fail.

One of the problems I see is that you can't select_single(AboutPage, objectName='aboutPage')
You have to do select_single(objectName='aboutPage')
Thats reported as bug #1337004.