qapi script: do not allow string discriminator

Since enum based discriminators provide better type-safety and
ensure that future qapi additions do not forget to adjust dependent
unions, forbid using string as discriminator from now on.

Signed-off-by: Wenchao Xia <wenchaoqemu@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/tests/Makefile b/tests/Makefile
index 88fca31..e146f81 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -145,7 +145,8 @@
         unclosed-list.json unclosed-object.json unclosed-string.json \
         duplicate-key.json union-invalid-base.json flat-union-no-base.json \
         flat-union-invalid-discriminator.json \
-        flat-union-invalid-branch-key.json flat-union-reverse-define.json)
+        flat-union-invalid-branch-key.json flat-union-reverse-define.json \
+        flat-union-string-discriminator.json)
 
 GENERATED_HEADERS += tests/test-qapi-types.h tests/test-qapi-visit.h tests/test-qmp-commands.h