Define persistence policy

[jvalkeal]

Now that we're trying to implement easier way for user to persist a machine, and as this first iteration is going to be based on interceptor it's probably a good idea to come up with a policy of some sort. Maybe user want to persist before/after state has been set or define if persisting failure should prevent state change to happen, etc. This would align with ideas of not force feeding some specific functionality. Also should think about introducing more meaningful interface around these ideas.