It is much less error-prone way. Previous approach didn't worked because
VMs weren't added here at 'domain-init'/'domain-loaded' event. And even
after adding such handlers it wasn't working because of
QubesOS/qubes-issues#1816.
It may be a little slower, but since it isn't used so often
(starting/stopping VM and reloading firewall), shouldn't be a problem.
c9cad71d03