Add backwards READ compatibility for the old libcontainer API

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)
fix compilation and panic

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #6808 from vieux/pr_6806

Add backwards READ compatibility for the old libcontainer API