Don't use get_core_device

clutter_device_manager_get_core_device calls XIGetClientPointer, which
requires a round-trip to the server. Since we do this on StWidget
creation, this means a full round-trip for every created StWidget.
Replace this with get_device with the ID of the VCP/VCK, since mutter
doesn't support MPX, and we know this is what the device is.