clean autodev dir on container exit

When `lxc.autodev = 1`, LXC creates automatically a `/dev/.lxc/<name>.<hash>` or `/dev/.lxc/user/<name>.<hash>` folder to put container's devices in so that they are visible from both the host and the container itself.

On container exit (be it normal or not), this folder was not cleaned which made "/dev" folder grow continuously.

This commit fixes this by adding a new `int lxc_delete_autodev(struct lxc_handler *handler)` called on exit from `static void lxc_fini(const char *name, struct lxc_handler *handler)`.