qapi: Fix to reject union command and event arguments

A command's or event's 'data' must be a struct type, given either as a
dictionary, or as struct type name.

Commit dd883c6 tightened the checking there, but not enough: we still
accept 'union'.  Fix to reject it.

We may want to support union types there, but we'll have to extend
qapi-commands.py and qapi-events.py for it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
diff --git a/scripts/qapi.py b/scripts/qapi.py
index 4879982..bbeae4d 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -499,7 +499,7 @@
 
     check_type(expr_info, "'data' for command '%s'" % name,
                expr.get('data'), allow_dict=True, allow_optional=True,
-               allow_metas=['union', 'struct'], allow_star=allow_star)
+               allow_metas=['struct'], allow_star=allow_star)
     returns_meta = ['union', 'struct']
     if name in returns_whitelist:
         returns_meta += ['built-in', 'alternate', 'enum']
@@ -517,7 +517,7 @@
     events.append(name)
     check_type(expr_info, "'data' for event '%s'" % name,
                expr.get('data'), allow_dict=True, allow_optional=True,
-               allow_metas=['union', 'struct'])
+               allow_metas=['struct'])
 
 def check_union(expr, expr_info):
     name = expr['union']