Support VHDX differencing images (ie images with backing)

The qemu vhdx driver does not support type 2 (differencing) vhdx images (usually stored with file extension .avhdx). This means that any hyperv images with snapshots are not supported by qemu-img. It would be great to be able to convert to a new qcow image from a backing + set of differencing images from hyperv, and/or convert an individual differencing vhdx image to a qcow2 image with a backing file specified.