lxc-destroy: deal with ephemeral containers

lxc-destroy: deal with ephemeral containers

- Ephemeral containers are destroyed on shutdown so we do not destroy them.
- Destroy ephemeral containers with clones: first destroy all the clones, then
  destroy the container.
- Ephemeral containers with snapshots cannot be easily handled but we can
  probably trust that no one will try to make snapshots of an ephemeral
  container.

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>