qapi: Do not cast function pointers

qapi: Do not cast function pointers

Using -fsanitize=undefined with Clang v18 causes an error if function
pointers are casted:

 qapi/qapi-clone-visitor.c:188:5: runtime error: call to function visit_type_SocketAddress through pointer to incorrect function type 'bool (*)(struct Visitor *, const char *, void **, struct Error **)'
 /tmp/qemu-ubsan/qapi/qapi-visit-sockets.c:487: note: visit_type_SocketAddress defined here
     #0 0x5642aa2f7f3b in qapi_clone qapi/qapi-clone-visitor.c:188:5
     #1 0x5642aa2c8ce5 in qio_channel_socket_listen_async io/channel-socket.c:285:18
     #2 0x5642aa2b8903 in test_io_channel_setup_async tests/unit/test-io-channel-socket.c:116:5
     #3 0x5642aa2b8204 in test_io_channel tests/unit/test-io-channel-socket.c:179:9
     #4 0x5642aa2b8129 in test_io_channel_ipv4 tests/unit/test-io-channel-socket.c:323:5
     ...

It also prevents enabling the strict mode of CFI which is currently
disabled with -fsanitize-cfi-icall-generalize-pointers.

The problematic casts are necessary to pass visit_type_T() and
visit_type_T_members() as callbacks to qapi_clone() and qapi_clone_members(),
respectively. Open-code these two functions to avoid the callbacks, and
thus the type casts.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2346
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240524-xkb-v4-3-2de564e5c859@daynix.com>
[thuth: Improve commit message according to Markus' suggestions]
Signed-off-by: Thomas Huth <thuth@redhat.com>