[Libguestfs] [PATCH nbdkit] v2v: Disable readahead for VMware curl sources too (RHBZ#1848862).

I'm still testing this fix, so let's hold off the review for the moment. Also it may be better to specifically identify problematic servers rather than disabling this for every curl source. eg. I suspect that the problem is the Java server used by VCenter, so we might think about only disabling readahead for that single case. Rich.