I'm thinking if support for XVA files could be added to qemu-img The file-format is well known (it's just a tar archive) and there are scripts that are able to convert an XVA file to a RAW image. (ie: https://gist.github.com/miebach/0433947bcf053de23159)
Running these script on their own is very time consuming, as you have to extract the XVA, convert any disk image from "single chunks" to a raw image and then use qemu-img to convert from raw to qcow. Maybe a native support will be able to skip some steps. (like the conversion from "single chunks" to raw and raw to qcow2)