serial: clear LSR.TEMT when populating the TSR

We never actually clear the TEMT (transmit sending register empty) flag when
populating the TSR.  We set the flag, but since it's never cleared, setting it
is sort of pointless..

I found this with a unit test case.
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/serial.c

@@ -332,6 +332,7 @@ static void serial_xmit(void *opaque)
         } else {
             s->tsr = s->thr;
             s->lsr |= UART_LSR_THRE;
+            s->lsr &= ~UART_LSR_TEMT;
         }
     }