Some time ago I wrote about ZFS storage driver for libvirt. At that time it worked on FreeBSD only because ZFS on Linux had some minor limitations, mainly some command line arguments for generating machine-readable output.
It will be available in 1.3.2, but if you're interested, you're welcome to play around with that right now checking out libvirt from git.
Again, most of the usage details are available in my older post. Also, feel free to poke me either on twitter or through email firstname.lastname@example.org (note the intentional typo). It could take some time to get a reply though.