qapi: Support downstream structs

Enhance the testsuite to cover downstream structs, including struct
members and base structs.  Update the generator to mangle the
struct 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 5f9af66..ca9b34c 100644
--- a/tests/qapi-schema/qapi-schema-test.json
+++ b/tests/qapi-schema/qapi-schema-test.json
@@ -110,3 +110,7 @@
 
 # test that we correctly compile downstream extensions
 { 'enum': '__org.qemu_x-Enum', 'data': [ '__org.qemu_x-value' ] }
+{ 'struct': '__org.qemu_x-Base',
+  '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' } }