[Libguestfs] virt-builder download issues

I'm having trouble downloading templates (ubuntu-18.04, among others) using virt-builder. The downloads get to about 50% (or 13, or 62) and then stall. wget easily downloads the files quickly. I'd be happy with working around this by manually placing the images in the cache directory, but I can't seem to figure out the appropriate placement. virt-builder doesn't find the cached file. Please send help, Thanks!