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>