device-manager-evdev: Set and unset the stage for the main seat too

When no input devices are available on startup the device manager might be fast enough to be constructed so that no default stage is set yet, and thus when main seat virtual devices are created they won't have a proper stage set. If then we plug a real device, the events that an input manager could generate won't be associated to any stage and thus won't be processed.

We need then ensure that when we update the stage for the device manager we (un)associate it also to the main seat devices.