nubus-bridge: make slot_available_mask a qdev property

This is to allow Macintosh machines to further specify which slots are available
since the number of addressable slots may not match the number of physical slots
present in the machine.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210924073808.1041-18-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>