anchor.diff:  
     createEmptyImage = {size, fullName}: ''
       mkdir $out
  -    diskImage=$out/image
  -    qemu-img create -f qcow $diskImage "${toString size}M"
  +    diskImage=$out/disk-image.qcow2
  +    qemu-img create -f qcow2 $diskImage "${toString size}M"
   
       mkdir $out/nix-support
       echo "${fullName}" > $out/nix-support/full-name
  
positive.code:     QEMU_OPTS = "-m ${toString (if attrs ? memSize then attrs.memSize else 256)}";
  
      preVM = ''
        diskImage=$(pwd)/image
        origImage=${attrs.diskImage}
        if test -d "$origImage"; then origImage="$origImage/image"; fi
        qemu-img create -b "$origImage" -f qcow $diskImage
  
        echo "$buildCommand" > cmd
  
  
negative.code:     fi
      diskImage="$1"
      if ! test -e "$diskImage"; then
        qemu-img create -b ${image}/disk-image.qcow2 -f qcow2 "$diskImage"
      fi
      export TMPDIR=$(mktemp -d)
      export out=/dummy
  