qapi: Command returning anonymous type doesn't work, outlaw

Reproducer: with

    { 'command': 'user_def_cmd4', 'returns': { 'a': 'int' } }

added to qapi-schema-test.json, qapi-commands.py dies when it tries to
generate the command handler function

    Traceback (most recent call last):
      File "/work/armbru/qemu/scripts/qapi-commands.py", line 359, in <module>
        ret = generate_command_decl(cmd['command'], arglist, ret_type) + "\n"
      File "/work/armbru/qemu/scripts/qapi-commands.py", line 29, in generate_command_decl
        ret_type=c_type(ret_type), name=c_name(name),
      File "/work/armbru/qemu/scripts/qapi.py", line 927, in c_type
        assert isinstance(value, str) and value != ""
    AssertionError

because the return type doesn't exist.

Simply outlaw this usage, and drop or dumb down test cases accordingly.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
diff --git a/scripts/qapi.py b/scripts/qapi.py
index bbeae4d..23c32fe 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -504,7 +504,7 @@
     if name in returns_whitelist:
         returns_meta += ['built-in', 'alternate', 'enum']
     check_type(expr_info, "'returns' for command '%s'" % name,
-               expr.get('returns'), allow_array=True, allow_dict=True,
+               expr.get('returns'), allow_array=True,
                allow_optional=True, allow_metas=returns_meta,
                allow_star=allow_star)