xen: request tx notification on tx completion

Otherwise, we never learn that transmission has completed and never
recycle ring entries.

This is still a little lame as we don't do any batching.

net/xenfront.cc

@@ -249,6 +249,7 @@ future<> xenfront_net_device::handle_tx_completions() {
         _tx_ring.entries.free_index(rsp.id);
     }
     _tx_ring.rsp_cons = prod;
+    _tx_ring._sring->rsp_event = prod + 1;
     return make_ready_future<>();
 }