Avoid temporarily unmounting the container when restarting it

Stopping the container will typicall cause it to unmount, to keep it mounted
over the stop/start cycle we aquire a temporary reference to it during this time.

This helps with https://github.com/dotcloud/docker/issues/4036

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)