q800: move CPU object into Q800MachineState

q800: move CPU object into Q800MachineState

Also change the instantiation of the CPU to use object_initialize_child()
followed by a separate realisation.
Restrict valid CPU types to m68040.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230621085353.113233-6-mark.cave-ayland@ilande.co.uk>
[lv: update commit message]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>