Change API to make all methods work on an opened image

[cgwalters]

This relates to #31 - basically it would make way more sense for us to expose methods on an opened image.

This would also help fix coreos/rpm-ostree#4213 because then we could impl Drop on it so that the image is automatically closed.

[cgwalters]

That said I've recently been thinking we actually should be using a better RPC API here, whether that's varlink or Cap'n Proto or something else. If we did that it might be more likely that we could hook in with an IPC-level API for ensuring resources are dropped.