tests: vmstate static checker: add size mismatch inside substructure

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

tests/vmstate-static-checker-data/dump2.json

@@ -909,7 +909,7 @@
                 "field": "kbd.keycodes",
                 "version_id": 0,
                 "field_exists": false,
-                "size": 4
+                "size": 2
               },
               {
                 "field": "head",