This device doesn't have fields to migrate. Be explicit
by using vmstate_qdev_no_state_to_migrate.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
 hw/misc/unimp.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/misc/unimp.c b/hw/misc/unimp.c
index bc4084d344..8a0db25a77 100644
--- a/hw/misc/unimp.c
+++ b/hw/misc/unimp.c
@@ -79,6 +79,7 @@ static void unimp_class_init(ObjectClass *klass, void *data)
     DeviceClass *dc = DEVICE_CLASS(klass);
 
     dc->realize = unimp_realize;
+    dc->vmsd = vmstate_qdev_no_state_to_migrate;
     device_class_set_props(dc, unimp_properties);
 }
 
-- 
2.21.3