sorry for the late response but it took some time for me to get a Linux system up and running to have KVM installed. At the end I had a Windows computer hosting Ubuntu inside VirtBox. And this Ubuntu VM was hosting my KVM system
Long story short, I had the same issue using x68 image file. However I worked around stating with a plain Debian installation and installed DietPi on top using PREP script.
I considered doing that after seeing the prep script on Github during my searches. Probably faster to resize via vbox image. Maybe when I get a free moment ill check out resizing a raw image and maybe do a tutorial. I have a feeling that’s where the problem was occurring. Kinda funny that we’re running Debian based DP and yet no KVM VM option! Oh well, no biggie!
Check out the forum link i posted, it’s pretty good to get it going from the vbox OVA file.