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

[Richard W.M. Jones]

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.