nubus-bridge: introduce separate NubusBridge structure

nubus-bridge: introduce separate NubusBridge structure

This is to allow the Nubus bridge to store its own additional state. Also update
the comment in the file header to reflect that nubus-bridge is not specific to
the Macintosh.

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: <20210924073808.1041-14-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>