virtio: use callback on empty in virtio_net

virtio_net uses a timer to free old transmitted packets, rather than
leaving callbacks enabled all the time.  If the host promises to
always notify us when the transmit ring is empty, we can free packets
at that point and avoid the timer.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>