qapi: Use 'alternate' to replace anonymous union

Previous patches have led up to the point where I create the
new meta-type "'alternate':'Foo'".  See the previous patches
for documentation; I intentionally split as much work into
earlier patches to minimize the size of this patch, but a lot
of it is churn due to testsuite fallout after updating to the
new type.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/tests/qapi-schema/alternate-base.json b/tests/qapi-schema/alternate-base.json
index dad7f02..66edc89 100644
--- a/tests/qapi-schema/alternate-base.json
+++ b/tests/qapi-schema/alternate-base.json
@@ -1,7 +1,6 @@
-# we reject anonymous union with base type
+# we reject alternate with base type
 { 'type': 'Base',
   'data': { 'string': 'str' } }
-{ 'union': 'MyUnion',
+{ 'alternate': 'Alt',
   'base': 'Base',
-  'discriminator': {},
   'data': { 'number': 'int' } }