serial: fix lost character after sysrq

After creating an automated regression test to test the sysrq
responses while running a linux image in qemu, I found that the
simulated uart was eating the character right after the sysrq about
75% of the time.

The problem is that the qemu sets the LSR_DR (data ready) bit on a
serial break.  The automated tests can send a break and the sysrq
character quickly enough that the qemu serial fifo has a real
character available. When there is valid character in the fifo, it
gets consumed by the serial driver in the guest OS.

The real hardware also appears to set the LSR_DR but always appears to
have a null byte in this condition.  This patch changes the qemu
behavior to match the tested characteristics of a real 16550 chip.
Signed-off-by: NJason Wessel <jason.wessel@windriver.com>

hw/serial.c

@@ -586,6 +586,8 @@ static int serial_can_receive(SerialState *s)
 static void serial_receive_break(SerialState *s)
 {
     s->rbr = 0;
+    /* When the LSR_DR is set a null byte is pushed into the fifo */
+    fifo_put(s, RECV_FIFO, '\0');
     s->lsr |= UART_LSR_BI | UART_LSR_DR;
     serial_update_irq(s);
 }