API: replying should be implemented on an Event

[Wenzel]

It's more natural to call reply on an Event than on the driver:

// old
drv.reply_event(&ev, EventReplyType::Continue)
// new
ev.reply(EventReplyType::Continue)

• [ ] find a way to hide the API reply_event in the Introspectable trait
• [ ] expose a new API in impl Event, so users should call this API instead

related: https://github.com/Wenzel/libmicrovmi/pull/49/files

thoughts @kylerky, @rageagainsthepc ?