I love OCaml as well, but haven't seen this before. It's interesting that libguestfs provides both object and module style interface and virt-builder seems to be using the object style:

http://libguestfs.org/guestfs-ocaml.3.html

Overall, great article and great code. Let's keep in mind that SML is typically taught to entry-level CS students (some of them with no programming experience) at universities like CMU, where often a term project might actually be an ML compiler in ML. It's not a terribly complex language and some limitations (which I am not going to state here) aside it has a great use to play.

It should be noted that XenServer also uses OCaml. While it's not really fair to compare Go with OCaml (the language roots are vastly different as are the philosophies -- aside from the idea that "static typing shouldn't mean typing so much you generate static electricity") both seem to do best in the "userland systems programming" niche -- that awkward place where C++ is needlessly painful, Java is ill-fitted, and python/etc... simply inappropriate.