Migration is specific to system emulation.

- Move the CPUClass::vmsd field to SysemuCPUOps,
- restrict VMSTATE_CPU() macro to sysemu,
- vmstate_dummy is now unused, remove it.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210517105140.1062037-16-f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>