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>

tests/qapi-schema/command-int.json

@@ -1,3 +1,2 @@
 # we reject collisions between commands and types
-{ 'command': 'int', 'data': { 'character': 'str' },
-  'returns': { 'value': 'int' } }
+{ 'command': 'int', 'data': { 'character': 'str' } }

tests/qapi-schema/nested-struct-data.json

@@ -1,4 +1,3 @@
 # inline subtypes collide with our desired future use of defaults
 { 'command': 'foo',
-  'data': { 'a' : { 'string' : 'str', 'integer': 'int' }, 'b' : 'str' },
-  'returns': {} }
+  'data': { 'a' : { 'string' : 'str', 'integer': 'int' }, 'b' : 'str' } }

tests/qapi-schema/nested-struct-returns.err

@@ -1 +0,0 @@
-tests/qapi-schema/nested-struct-returns.json:2: Member 'a' of 'returns' for command 'foo' should be a type name

tests/qapi-schema/nested-struct-returns.json

@@ -1,3 +0,0 @@
-# inline subtypes collide with our desired future use of defaults
-{ 'command': 'foo',
-  'returns': { 'a' : { 'string' : 'str', 'integer': 'int' }, 'b' : 'str' } }

tests/qapi-schema/returns-dict.err

@@ -0,0 +1 @@
+tests/qapi-schema/returns-dict.json:2: 'returns' for command 'oops' should be a type name

tests/qapi-schema/returns-dict.json

@@ -0,0 +1,2 @@
+# we reject inline struct return type
+{ 'command': 'oops', 'returns': { 'a': 'str' } }