Add way to commit changes to image

[phlmn]

Either call commit on running vm via REPL or think of maintaining git like snapshots.

[phlmn]

In favor of #5 we create a new image for each change, so images are immutable and can be referenced by sessions. Sadly one cannot use qemu's commit to create a new image. We could add a convenient way to promote a session to a base image, but it would be import that the session is not too old, since it is based on the recent image at the time of creating the session.

I think for now it's fine to have to start the vm --edit-base-image for creating a new version.

[phlmn]

Commiting from a running vm is flawed anyway. At least windows might boot into recovery mode when it is not shut down properly