Instantiate ListOpts

Unfortunately, compiling and running systemd-docker using the vendored deps right now results in a panic. Apparently, I ended up inadvertently vendoring in a newer version of `github.com/docker/docker/opts` than was used to build systemd-docker originally. This upstream commit [0] is the culprit. Updating the usage of ListOpts fixes the panic.

[0] https://github.com/docker/docker/commit/6200002669874f3314856527fecd0c004060913c