/var/lib/qubes/appvms/<yourHVM>/ has no root.img file in it. the only file in the directory is icon.png
How do I resize an HVM disk on 4.0rc1
How are HVM disks resized on Qubes 4.0rc1?
Resizing disks is now done in GUI from the Qubes drop-down menu in dom0 under [MyHVM] > VM Settings