hw/char/serial: Only retry if qemu_chr_fe_write returns 0

Only retry on serial_xmit if qemu_chr_fe_write returns 0, as this is the
only recoverable error.

Retrying with any other scenario, in addition to being a waste of CPU
cycles, can compromise the Guest stability if by the vCPU issuing the
write and the main loop thread are, by chance or explicit pinning,
running on the same pCPU.

Previous discussion:

https://lists.nongnu.org/archive/html/qemu-devel/2018-05/msg06998.htmlSigned-off-by: NSergio Lopez <slp@redhat.com>
Message-Id: <1528185295-14199-1-git-send-email-slp@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/char/serial.c

@@ -260,7 +260,7 @@ static void serial_xmit(SerialState *s)
         if (s->mcr & UART_MCR_LOOP) {
             /* in loopback mode, say that we just received a char */
             serial_receive1(s, &s->tsr, 1);
-        } else if (qemu_chr_fe_write(&s->chr, &s->tsr, 1) != 1 &&
+        } else if (qemu_chr_fe_write(&s->chr, &s->tsr, 1) == 0 &&
                    s->tsr_retry < MAX_XMIT_RETRY) {
             assert(s->watch_tag == 0);
             s->watch_tag =