vhost: restore avail index from vring used index on disconnection

vhost_virtqueue_stop() gets avail index value from the backend,
except if the backend is not responding.

It happens when the backend crashes, and in this case, internal
state of the virtio queue is inconsistent, making packets
to corrupt the vring state.

With a Linux guest, it results in following error message on
backend reconnection:

[   22.444905] virtio_net virtio0: output.0:id 0 is not a head!
[   22.446746] net enp0s3: Unexpected TXQ (0) queue failure: -5
[   22.476360] net enp0s3: Unexpected TXQ (0) queue failure: -5

Fixes: 283e2c2adcb8 ("net: virtio-net discards TX data after link down")
Cc: qemu-stable@nongnu.org
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
