libcontainer: Create dirs/files as needed for bind mounts

If you specify a bind mount in a place that doesn't have a file yet
we create that (and parent directories). This is needed because
otherwise you can't put volumes on e.g. /dev or /run, as those
would get covered by tmpfs mounts.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)