build-support/vm: fix makeImageTestScript

`qemu-img create` now requires an additional `-F <second_image_format>` flag.

https://github.com/qemu/qemu/blob/master/docs/about/removed-features.rst#qemu-img-backing-file-without-format-removed-in-61

pkgs/build-support/vm/default.nix

···
     fi
     diskImage="$1"
     if ! test -e "$diskImage"; then
-      ${qemu}/bin/qemu-img create -b ${image}/disk-image.qcow2 -f qcow2 "$diskImage"
+      ${qemu}/bin/qemu-img create -b ${image}/disk-image.qcow2 -f qcow2 -F qcow2 "$diskImage"
     fi
     export TMPDIR=$(mktemp -d)
     export out=/dummy