lguest: don't force VIRTIO_F_NOTIFY_ON_EMPTY

VIRTIO_F_NOTIFY_ON_EMPTY indicates to the Guest that we will hit them with
an interrupt every time the xmit queue is emptied.

Because it results in lots of tx interrupts, modern Guests probably don't
want it, so let's only force it when they accept the option.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>