qapi: Require boxed for conditional command and event arguments

The C code generator fails to honor 'if' conditions of command and
event arguments.

For instance, tests/qapi-schema/qapi-schema-test.json has

    { 'event': 'TEST_IF_EVENT',
      'data': { 'foo': 'TestIfStruct',
		'bar': { 'type': ['str'], 'if': 'TEST_IF_EVT_ARG' } },
      'if': { 'all': ['TEST_IF_EVT', 'TEST_IF_STRUCT'] } }

Generated tests/test-qapi-events.h fails to honor the TEST_IF_EVT_ARG
condition:

    #if defined(TEST_IF_EVT) && defined(TEST_IF_STRUCT)
    void qapi_event_send_test_if_event(TestIfStruct *foo, strList *bar);
    #endif /* defined(TEST_IF_EVT) && defined(TEST_IF_STRUCT) */

Only uses so far are in tests/.

We could fix the generator to emit something like

    #if defined(TEST_IF_EVT) && defined(TEST_IF_STRUCT)
    void qapi_event_send_test_if_event(TestIfStruct *foo
    #if defined(TEST_IF_EVT_ARG)
                    , strList *bar
    #endif
                    );
    #endif /* defined(TEST_IF_EVT) && defined(TEST_IF_STRUCT) */

Ugly.  Calls become similarly ugly.  Not worth fixing.

Conditional arguments work fine with 'boxed': true, simply because
complex types with conditional members work fine.  Not worth breaking.

Reject conditional arguments unless boxed.

Move the tests cases covering unboxed conditional arguments out of
tests/qapi-schema/qapi-schema-test.json.  Cover boxed conditional
arguments there instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230316071325.492471-15-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

tests/qapi-schema/meson.build

@@ -27,6 +27,8 @@ schemas = [
   'args-bad-boxed.json',
   'args-boxed-anon.json',
   'args-boxed-string.json',
+  'args-if-implicit.json',
+  'args-if-unboxed.json',
   'args-int.json',
   'args-invalid.json',
   'args-member-array-bad.json',