qapi: Document shortcoming with union 'data' branch

Add a FIXME to remind us to fully audit whether removing the
'void *data' branch of each qapi union type can be done safely.

Backports commit ca56a822dd538017715345cbbe1f8829e0cc2742 from qemu

qemu/scripts/qapi-types.py

@@ -217,6 +217,14 @@ struct %(name)s
 ''',
                      discriminator_type_name=c_name(discriminator_type_name))
 
+    # 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
+    # failed; but since we 0-initialize structs, and cannot tell what
+    # branch of the union is in use if the discriminator is invalid, there
+    # 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?
     ret += mcgen('''
     union { /* union tag is @%(c_name)s */
         void *data;