[Libguestfs] [PATCH v3 4/4] v2v: Add --print-estimate option to print copy size

I rethought this again, as I think that it's a dangerous assumption to bake qemu-img measure output into our API. This patch series runs qemu-img measure behind the scenes, but then parses the output and sums it to a single number which we print. Doing that required a bit of reworking, moving the Jansson [JSON parser] bindings from virt-builder into the common directory and a couple of other minor changes, hence this has grown to 4 commits. Rich.