macfb: add VMStateDescription fields for display type and VBL timer

macfb: add VMStateDescription fields for display type and VBL timer

These fields are required in the migration stream to restore macfb state
correctly.

Note this is a migration break, but since there are upcoming incompatible changes
for the q800 machine (and migration does not even succeed without these patches)
then this is not an issue.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20220305155530.9265-5-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>