After checkpointing with one engine users might forget to add the engine VM option during restore; this would lead to failure but the message that the engine generates would be confusing (e.g. missing file in the checkpoint directory, or invalid format of some file).

JVM can create a simple `engine` file to the directory with image, writing the engine name used, and on restore check that it matches.

Non-goal: In the future we could use this file to load the engine default (rather than the current `criuengine`); this needs to be thoroughly considered, though. One counter-argument would be if the checkpoint location is not a directory (e.g. network location) and JVM could not recognize that, users would need to set the engine explicitly, causing some asymmetry in restore VM options.