I was digging QEMU's code base to see how 'qemu-img convert' ends up
calling fsync(2).  See the number of calls involved:

  - In block/io.c, we begin with: bdrv_flush()  -->
    bdrv_flush_co_entry() --> bdrv_co_flush().

  - Then:  bdrv_co_flush() --> bdrv_co_flush_to_disk() -->
    raw_co_flush_to_disk(), which sets handle_aiocb_flush() as a
    callback function.

  - And finally: handle_aiocb_flush() --> qemu_fdatasync() --> fsync(2).