With commit d4c19cde("virtio-serial: add missing virtio_detach_element() call"),
when virtio serial is throttled and closed by host, port->elem should be discard with virtqueue_push,
otherwise virtqueue will not rewind, guest will blocked finally and cannot write anymore data.

It can be reproduced with following steps:
Create a vm with virtio-serial device through libvirt:

    <channel type='unix'>
      <source mode='bind' path='/tmp/test'/>
      <target type='virtio' name='com.test.channel.0'/>
      <address type='virtio-serial' controller='0' bus='0' port='1'/>
    </channel>

Host run:
watch -n0.5 'timeout 0.5 nc -U /tmp/test'

Guest run:
hexdump -C -v /dev/zero > /dev/vport1p1

After sometime, guest can not write any data to serial.
We can see vq->last_avail_idx - vq->used_idx = 128 with gdb.

(gdb) p *(struct VirtQueue *) 0x565161c5c788
$4 = {vring = {num = 128, num_default = 128, align = 4096, desc = 4316049408, avail = 4316051456, used = 4316051776,
    caches = 0x7fc530067e60}, used_elems = 0x565161c88dc0, last_avail_idx = 7929, last_avail_wrap_counter = true,
  shadow_avail_idx = 7929, shadow_avail_wrap_counter = true, used_idx = 7801, used_wrap_counter = true, signalled_used = 7801,
  signalled_used_valid = true, notification = true, queue_index = 5, inuse = 0, vector = 1,
  handle_output = 0x56515da8aea0 <handle_output>, handle_aio_output = 0x0, vdev = 0x565161c54e30, guest_notifier = {rfd = 0,
    wfd = 0}, host_notifier = {rfd = 74, wfd = 74}, host_notifier_enabled = true, node = {le_next = 0x565161c5c6f0,
    le_prev = 0x565161c5c8a8}}

Fixes: d4c19cde("virtio-serial: add missing virtio_detach_element() call")
Signed-off-by: Sun Feng <loyou85@gmail.com>
---
 hw/char/virtio-serial-bus.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c
index dd619f0..30b00a4 100644
--- a/hw/char/virtio-serial-bus.c
+++ b/hw/char/virtio-serial-bus.c
@@ -151,7 +151,7 @@ static void discard_vq_data(VirtQueue *vq, VirtIODevice *vdev)
 static void discard_throttle_data(VirtIOSerialPort *port)
 {
     if (port->elem) {
-        virtqueue_detach_element(port->ovq, port->elem, 0);
+        virtqueue_push(port->ovq, port->elem, 0);
         g_free(port->elem);
         port->elem = NULL;
     }
-- 
2.7.4