serial: reset state at startup

When a serial port is started, its initial state is all zero.  Make
it consistent with reset state instead.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/char/serial.c

@@ -833,6 +833,7 @@ void serial_realize_core(SerialState *s, Error **errp)
                           serial_event, s);
     fifo8_create(&s->recv_fifo, UART_FIFO_LENGTH);
     fifo8_create(&s->xmit_fifo, UART_FIFO_LENGTH);
+    serial_reset(s);
 }
 
 void serial_exit_core(SerialState *s)