virtio-net: purge outstanding packets when starting vhost

virtio-net: purge outstanding packets when starting vhost

whenever we start vhost, virtio could have outstanding packets
queued, when they complete later we'll modify the ring
while vhost is processing it.

To prevent this, purge outstanding packets on vhost start.

Cc: qemu-stable@nongnu.org
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 086abc1ccd0fa5103345adda819e6c6436949579)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>