Rewrite the existing VMSTATE_FIFO8 macro to use VMSTATE_FIFO8_TEST as per the
standard pattern in include/migration/vmstate.h.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210128221728.14887-3-mark.cave-ayland@ilande.co.uk>