exec: Assert CPU migration is not used on user-only build

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200522172510.25784-9-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

exec.c

@@ -946,7 +946,9 @@ void cpu_exec_realizefn(CPUState *cpu, Error **errp)
 
     qemu_plugin_vcpu_init_hook(cpu);
 
-#ifndef CONFIG_USER_ONLY
+#ifdef CONFIG_USER_ONLY
+    assert(cc->vmsd == NULL);
+#else /* !CONFIG_USER_ONLY */
     if (qdev_get_vmsd(DEVICE(cpu)) == NULL) {
         vmstate_register(NULL, cpu->cpu_index, &vmstate_cpu_common, cpu);
     }