qapi: Fix generated code when flat union has member 'kind'

A flat union's tag member gets renamed to 'kind' in the generated
code.  Breaks when another member named 'kind' exists.

Example, adapted from qapi-schema-test.json:

    { 'struct': 'UserDefUnionBase',
      'data': { 'kind': 'str', 'enum1': 'EnumOne' } }

We generate:

    struct UserDefFlatUnion
    {
        EnumOne kind;
        union {
            void *data;
            UserDefA *value1;
            UserDefB *value2;
            UserDefB *value3;
        };
        char *kind;
    };

Kill the silly rename.

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

scripts/qapi-types.py

@@ -200,11 +200,12 @@ def generate_union(expr, meta):
     ret = mcgen('''
 struct %(name)s
 {
-    %(discriminator_type_name)s kind;
+    %(discriminator_type_name)s %(discriminator)s;
     union {
         void *data;
 ''',
                 name=name,
+                discriminator=c_name(discriminator or 'kind'),
                 discriminator_type_name=c_name(discriminator_type_name))
 
     for key in typeinfo: