I repeat -
I think you have not read those instructions properly. There is nothing
there about converting to iso, and therefore nothing to make you think
you need to do this in dom0.
Those instructions show you how to convert the vmdk to a raw image, then
copy the raw image to dom0, so that you can use it as the basis for a
new root image for the HVM.
There is nothing about converting to an iso.
There is nothing Qubes specific here.
What you say you want to do is produce an iso from the vmdk.
If you know how to do this in Windows, why do you not install a Windows
HVM with adequate space, and do it there?
Or, look at the online guides, and run it in an adequately sized Linux qube or
standalone?