Should remove old xenstore entries about unknown interfaces on startup

When interfaces/addresses reported to Xenstore are suppressed while the service is not running, those stall data remain.

We could be more clever and only remove the old entries, but it would suddenly become more complicated, so let's just remove all entries from previous agent, and we're already ready to repopulate immediately. Opened #15 to get that smart (not a priority but can be a challenging newcomer project)