qapi: Finish converting to new qapi union layout

We have two issues with our qapi union layout:
1) Even though the QMP wire format spells the tag 'type', the
C code spells it 'kind', requiring some hacks in the generator.
2) The C struct uses an anonymous union, which places all tag
values in the same namespace as all non-variant members. This
leads to spurious collisions if a tag value matches a non-variant
member's name.

This patch is the back end for a series that converts to a
saner qapi union layout. Now that all clients have been
converted to use 'type' and 'obj->u.value', we can drop the
temporary parallel support for 'kind' and 'obj->value'.

Given a simple union qapi type:

{ 'union':'Foo', 'data': { 'a':'int', 'b':'bool' } }

this is the overall effect, when compared to the state before
this series of patches:

| struct Foo {
|- FooKind kind;
|- union { /* union tag is @kind */
|+ FooKind type;
|+ union { /* union tag is @type */
| void *data;
| int64_t a;
| bool b;
|- };
|+ } u;
| };

The testsuite still contains some examples of artificial restrictions
(see flat-union-clash-type.json, for example) that are no longer
technically necessary, now that there is no longer a collision between
enum tag values and non-variant member names; but fixing this will be
done in later patches, in part because some further changes are required
to keep QAPISchema*.check() from asserting. Also, a later patch will
add a reservation for the member name 'u' to avoid a collision between a
user's non-variant names and our internal choice of C union name.

Note, however, that we do not rename the generated enum, which
is still 'FooKind'. A further patch could generate implicit
enums as 'FooType', but while the generator already reserved
the '*Kind' namespace (commit 4dc2e69), there are already QMP
constructs with '*Type' naming, which means changing our
reservation namespace would have lots of churn to C code to
deal with a forced name change.

Backports commit e4ba22b31943ab02373359555bd7bcd66442632f from qemu

qemu/scripts/qapi-types.py

@@ -145,23 +145,10 @@ struct %(c_name)s {
     if base:
         ret += gen_struct_fields([], base)
     else:
-        # TODO As a hack, we emit both 'kind' and 'type'. Ultimately, we
-        # want to use only 'type', but the conversion is large enough to
-        # require staging over several commits.
-        ret += mcgen('''
-    union {
-        %(c_type)s kind;
-        %(c_type)s type;
-    };
-''',
-                     c_type=c_name(variants.tag_member.type.name))
+        ret += gen_struct_field(variants.tag_member.name,
+                                variants.tag_member.type,
+                                False)
 
-    # TODO As a hack, we emit the union twice, once as an anonymous union
-    # and once as a named union.  Ultimately, we want to use only the
-    # named union version (as it avoids conflicts between tag values as
-    # branch names competing with non-variant QMP names), but the conversion
-    # is large enough to require staging over several commits.
-    tmp = ''
     # FIXME: What purpose does data serve, besides preventing a union that
     # has a branch named 'data'? We use it in qapi-visit.py to decide
     # whether to bypass the switch statement if visiting the discriminator
@@ -170,7 +157,7 @@ struct %(c_name)s {
     # should not be any data leaks even without a data pointer.  Or, if
     # 'data' is merely added to guarantee we don't have an empty union,
     # shouldn't we enforce that at .json parse time?
-    tmp += mcgen('''
+    ret += mcgen('''
     union { /* union tag is @%(c_name)s */
         void *data;
 ''',
@@ -179,14 +166,12 @@ struct %(c_name)s {
     for var in variants.variants:
         # Ugly special case for simple union TODO get rid of it
         typ = var.simple_union_type() or var.type
-        tmp += mcgen('''
+        ret += mcgen('''
         %(c_type)s %(c_name)s;
 ''',
                      c_type=typ.c_type(),
                      c_name=c_name(var.name))
 
-    ret += tmp
-    ret += '    ' + '\n    '.join(tmp.split('\n'))
     ret += mcgen('''
     } u;
 };