hw/char: a question about watch callback function in serial

[LIU Zhiwei]

    Hi folks, 
    
    I see many UART implementations have a G_IO_OUT | G_IO_HUP  callback
    function.
    
    In hw/serial.c, it is serial_watch_cb, setting by the following
    code,
    
  s->watch_tag = qemu_chr_fe_add_watch(&s->chr, G_IO_OUT | G_IO_HUP,
                      serial_watch_cb, s);

    In hw/candence_uart.c, it is cadence_uart_xmit, setting by the
    following code,
    
        guint r = qemu_chr_fe_add_watch(&s->chr, G_IO_OUT | G_IO_HUP,
                                        cadence_uart_xmit, s);
    
    
    I tried to call it with booting a Linux, but the interface will
    never be called. 
    
    Can someone give a reasonable answer why needs this interface, or a
    way to call it.
    
    Best Regards,
    Zhiwei