Device mapper was creating an invalid cgroup name.

This was breaking docker top CONTAINER

Docker-DCO-1.1-Signed-off-by: Daniel Walsh <dwalsh@redhat.com> (github: rhatdan)
docker '' causes a golang crash.

This patch fixes the problem.

Docker-DCO-1.1-Signed-off-by: Daniel Walsh <dwalsh@redhat.com> (github: rhatdan)