block/file-posix: Remove a deprecation warning on macOS 12

When building on macOS 12 we get:

  block/file-posix.c:3335:18: warning: 'IOMasterPort' is deprecated: first deprecated in macOS 12.0 [-Wdeprecated-declarations]
      kernResult = IOMasterPort( MACH_PORT_NULL, &masterPort );
                   ^~~~~~~~~~~~
                   IOMainPort

Replace by IOMainPort, redefining it to IOMasterPort if not available.

Suggested-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed by: Cameron Esfahani <dirty@apple.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Tested-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>