nubus: move slot bitmap checks from NubusDevice realize() to BusClass check_address()

Allow Nubus to manage the slot allocations itself using the BusClass check_address()
virtual function rather than managing this during NubusDevice realize().

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: <20210924073808.1041-6-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>