qapi-types: Convert to QAPISchemaVisitor, fixing flat unions

Fixes flat unions to get the base's base members.  Test case is from
commit 2fc0043, in qapi-schema-test.json:

    { 'union': 'UserDefFlatUnion',
      'base': 'UserDefUnionBase',
      'discriminator': 'enum1',
      'data': { 'value1' : 'UserDefA',
                'value2' : 'UserDefB',
                'value3' : 'UserDefB' } }

    { 'struct': 'UserDefUnionBase',
      'base': 'UserDefZero',
      'data': { 'string': 'str', 'enum1': 'EnumOne' } }

    { 'struct': 'UserDefZero',
      'data': { 'integer': 'int' } }

Patch's effect on UserDefFlatUnion:

     struct UserDefFlatUnion {
         /* Members inherited from UserDefUnionBase: */
    +    int64_t integer;
         char *string;
         EnumOne enum1;
         /* Own members: */
         union { /* union tag is @ENUM1 */
             void *data;
             UserDefA *value1;
             UserDefB *value2;
             UserDefB *value3;
         };
     };

Flat union visitors remain broken.  They'll be fixed next.

Code is generated in a different order now, but that doesn't matter.

The two guards QAPI_TYPES_BUILTIN_STRUCT_DECL and
QAPI_TYPES_BUILTIN_CLEANUP_DECL are replaced by just
QAPI_TYPES_BUILTIN.

Two ugly special cases for simple unions now stand out like sore
thumbs:

1. The type tag is named 'type' everywhere, except in generated C,
   where it's 'kind'.

2. QAPISchema lowers simple unions to semantically equivalent flat
   unions.  However, the C generated for a simple unions differs from
   the C generated for its equivalent flat union, and we therefore
   need special code to preserve that pointless difference for now.

Mark both TODO.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Daniel P. Berrange <[email protected]>
Reviewed-by: Eric Blake <[email protected]>

docs/qapi-code-gen.txt

@@ -545,7 +545,7 @@ Example:
     $ cat qapi-generated/example-qapi-types.c
 [Uninteresting stuff omitted...]
 
-    void qapi_free_UserDefOneList(UserDefOneList *obj)
+    void qapi_free_UserDefOne(UserDefOne *obj)
     {
         QapiDeallocVisitor *md;
         Visitor *v;
@@ -556,12 +556,11 @@ Example:
 
         md = qapi_dealloc_visitor_new();
         v = qapi_dealloc_get_visitor(md);
-        visit_type_UserDefOneList(v, &obj, NULL, NULL);
+        visit_type_UserDefOne(v, &obj, NULL, NULL);
         qapi_dealloc_visitor_cleanup(md);
     }
 
-
-    void qapi_free_UserDefOne(UserDefOne *obj)
+    void qapi_free_UserDefOneList(UserDefOneList *obj)
     {
         QapiDeallocVisitor *md;
         Visitor *v;
@@ -572,7 +571,7 @@ Example:
 
         md = qapi_dealloc_visitor_new();
         v = qapi_dealloc_get_visitor(md);
-        visit_type_UserDefOne(v, &obj, NULL, NULL);
+        visit_type_UserDefOneList(v, &obj, NULL, NULL);
         qapi_dealloc_visitor_cleanup(md);
     }
     $ cat qapi-generated/example-qapi-types.h
@@ -585,24 +584,24 @@ Example:
 
     typedef struct UserDefOne UserDefOne;
 
-    typedef struct UserDefOneList {
+    typedef struct UserDefOneList UserDefOneList;
+
+    struct UserDefOne {
+        int64_t integer;
+        char *string;
+    };
+
+    void qapi_free_UserDefOne(UserDefOne *obj);
+
+    struct UserDefOneList {
         union {
             UserDefOne *value;
             uint64_t padding;
         };
         struct UserDefOneList *next;
-    } UserDefOneList;
-
-
-[Functions on built-in types omitted...]
-
-    struct UserDefOne {
-        int64_t integer;
-        char *string;
     };
 
     void qapi_free_UserDefOneList(UserDefOneList *obj);
-    void qapi_free_UserDefOne(UserDefOne *obj);
 
     #endif