macfb: add qdev property to specify display type

Since the available resolutions and colour depths are determined by the attached
display type, add a qdev property to allow the display type to be specified.

The main resolutions of interest are high resolution 1152x870 with 8-bit colour
and SVGA resolution up to 800x600 with 24-bit colour so update the q800 machine
to allow high resolution mode if specified and otherwise fall back to SVGA.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20211007221253.29024-9-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>