qapi: Support downstream simple unions

Enhance the testsuite to cover downstream simple unions, including
when a union branch is a downstream name.  Update the generator to
mangle the union names in the appropriate places.

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/tests/qapi-schema/qapi-schema-test.json b/tests/qapi-schema/qapi-schema-test.json
index ca9b34c..6416d85 100644
--- a/tests/qapi-schema/qapi-schema-test.json
+++ b/tests/qapi-schema/qapi-schema-test.json
@@ -114,3 +114,4 @@
   'data': { '__org.qemu_x-member1': '__org.qemu_x-Enum' } }
 { 'struct': '__org.qemu_x-Struct', 'base': '__org.qemu_x-Base',
   'data': { '__org.qemu_x-member2': 'str' } }
+{ 'union': '__org.qemu_x-Union1', 'data': { '__org.qemu_x-branch': 'str' } }