Make VMs on bootable block devices

I would like to create a VM thats fully “self-contained” on a block device to possibly boot on them bare-metal or in other hypervisers.
I know how to mount other block devices to a VM but despite my best efforts I havent been able to make the drives bootable.
Here is what I get when I boot off the drive I tried installing Windows 10 on using Qubes :

(this boot was attempted with kvm, i440fx chipset, bios)

What am I missing ?