[lxc]: support up-to-date lxc-ps versions

Since lxc 0.8.0 lxc-ps has enforced the '--' option on ps args

https://github.com/lxc/lxc/commit/a3812bf0c756029b0c7b8e3ca74c57728a791ab1

Also update the script to query for the lxc name directly rather than attempting to get all processes (which is deprecated)