pybind/rbd: ensure image is open before permitting operations

[trociny]

Fixes: https://tracker.ceph.com/issues/44610
Signed-off-by: Mykola Golub mgolub@suse.com

Checklist

• References tracker ticket
• Updates documentation if necessary
• Includes tests for new functionality or reproducer for bug

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[trociny]

I suppose a much cleaner solution would be just to remove close method (forbid public access) but it would break API.

[dillaman]

lgtm -- I probably would have used a decorator, but this matches the rados style.

[trociny]

@dillaman I tried with a decorator and it really looks nicer for me, so I updated the PR.