Booting an Ubuntu LTS cloud-init QEMU VM inside a Proxmox VM

I needed a quick throwaway Ubuntu VM for some testing, so I reached for the classic combo: an Ubuntu cloud image, a local cloud-init NoCloud datasource served over HTTP, and a bare qemu-system-x86_64 invocation. Nothing fancy, I've done this a dozen times before - just that the last time was ages ago.. My first attempt looked like this: root@vm:~# qemu-system-x86_64 -net nic -net user -machine accel=kvm:tcg -m 512 -nographic -hda noble-server-cloudimg-amd64.img -smbios type=1,serial=ds='nocloud;s=http://127.0.0.1:8000/' And of course, it didn't just work. Last time I did was ages ago and directly on the hardware of my rootserver, not my local Proxmox server. As it turned out, I had two separate problems in this command, and to make things even more interesting: Everything is executed inside a Debian 13 VM, which itself lives on top of Proxmox. A VM inside a VM. So every weirdness I hit had to be checked twice. Once for "Is this a QEMU problem?" and once for "Is this a…