Instantiate the PS2 mouse device within PL050MouseState using
object_initialize_child() in pl050_mouse_init() and realize it in
pl050_mouse_realize() accordingly.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Tested-by: Helge Deller <deller@gmx.de>
Acked-by: Helge Deller <deller@gmx.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20220712215251.7944-12-mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>