g364fb: use RAM memory region for framebuffer

Since the migration stream is already broken, we can use this opportunity to
change the framebuffer so that it is migrated as a RAM memory region rather
than as an array of bytes.

In particular this helps the output of the analyze-migration.py tool which
no longer contains a huge array representing the framebuffer contents.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210625163554.14879-2-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>