Comment 5 for bug 1988527

There is sadly no shortage of options to set the accelerator and they keep changing which might cause subtle unexpected changes on upgrades.

If you had this working before it might have been by some now deprecated and incompatible options. For example some args used to set accelerators have changed in recent years:
-machine accel= + -accel at the same time are now incompatible
-accel tcg -accel kvm => used to only do kvm, now will do tcg:kvm

That is in since qemu v5.0.0 which translates to 20.10 and later.

There is also "-enable-kvm" which overrules any of the former.
As I said, there are too many options controlling the same

"kvm" used to be a wrapper script which added -enable-kvm, but now is just a symlink - due to qemu internally setting to kvm:tcg now it should have the same behavior, with one twist.
In the past in an env that can only do TCG but not KVM it would have:
$ kvm ...
- old - used -enable-kvm: abort, can't initialize KVM
- new - sets kvm:tcg: try KVM, fall back to TCG

That was considered an improvement as it works in more cases, but it might have led in your case to a silent downgrade to tcg?
In any case, specifying explicitly what you want should be best.

I see autopkgtest uses qemu-system-*, so I'd expect that explicitly setting accel to kvm:tcg should be helpful.
But I'd recommend to not add it as further "-machine" but extend the existing one (if any).