lxd fails in unpredictable ways when used with an unsupported "future" guest

[basak]

If an older lxd tries to use a newer image, it may fail in strange ways after the container is running, even though it did not complain when creating or starting the container.

See #13806 for the background. Today's lxd presumably has the same problem then, which will manifest when the next incompatible thing happens inside guests which the next lxd release will not be able to support.

Actual results

The issue is that this results in a really poor UX. lxd appears to be able to run a future container fine, but then breaks in some unexpected way.

Expected results

Instead, if lxd isn't going to be able to work with some future image feature an older lxd that cannot support it should refuse to start such a container by default with a sensible error message (though a --force type option would be fine).

Suggestion for implementation

lxd could be taught to be aware of some kind of "compatibility level" of images, and the images we produce to contain metadata such that it can implement the above.