virtio-net: unify vhost-net start/stop Move all of vhost-net start/stop logic to a single routine, and call it from everywhere. Additionally, start/stop vhost-net on link up/down: we should not transmit anything if user asked us to put the link down. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Acked-by: Alex Williamson <alex.williamson@redhat.com>