libhyperkit doesn't build with recent versions of ocaml-qcow

[copumpkin]

It looks like the most recent version it works with is 0.6.0. They're up to 0.9.5, but the Qcow.Make functor now takes two arguments instead of one, since it's parametrized over time as well. I'm not sure much else needs to be done to support the new version.

I ran into this when building docker-machine-driver-xhyve if that helps.

[copumpkin]

Oh, I see that they've fixed it in upstream hyperkit now: https://github.com/docker/hyperkit/blob/master/src/lib/mirage_block_ocaml.ml#L11