Cleanup implicit dependencies between drive handler and jailer

As detailed in this comment: https://github.com/firecracker-microvm/firecracker-containerd/pull/339#discussion_r352941338

There are some delicate dependencies between the stubdrive paths created by drive handler and where each jailer assumes the paths are located. One possible improvement would be to have the jailer interface expose a method that tells the drive handler what path to use for a given drive, which should make the implicit dependency more explicit.