cuda: introduce CUDAState parameter to get_counter()

This will be required shortly and also happens to match nicely with the
corresponding signature for set_counter().

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>