Migration is specific to system emulation.

Restrict current DeviceClass::vmsd to sysemu using #ifdef'ry,
and assert in cpu_exec_realizefn() that dc->vmsd not set under
user emulation.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210517105140.1062037-12-f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>