Create mountable compressed disk image

1. Install tools

   apt install -y qemu-utils libguestfs-tools

2. Sparsify the disk

   virt-sparsify -v --in-place disk.img

3. Convert the disk to dynamic VHD

   qemu-img convert -p -o subformat=dynamic -f raw -O vpc disk.img disk.vhd

for i in *.img; do virt-sparsify -v --in-place $i; done
for i in *.img; do echo $i; qemu-img convert -p -o subformat=dynamic -f raw -O vpc "$i" "${i%%.img}.vhd"; done

1. http://libguestfs.org/virt-sparsify.1.html
2. https://docs.openstack.org/image-guide/convert-images.html