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.

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>

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