next-cube: move SCSI CSRs from next-pc to the next-scsi device

next-cube: move SCSI CSRs from next-pc to the next-scsi device

The SCSI CSRs are located within the SCSI subsystem of the NeXT PC (Peripheral
Contoller) which is now modelled as a separate QEMU device. Add a new memory
region subregion to contain the SCSI CSRs that simply store and retrieve the
register values.

Add a new VMStateDescription for the next-scsi device to enable the SCSI CSRs
to be migrated.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>
Message-ID: <20241222130012.1013374-8-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>