Describe the bug
When trying to generate an EC2 AMI, qemu fails with:

qemu-system-x86_64: CPU model ‘host’ requires KVM

This has been reported by someone else in the NixOS Discourse too: https://discourse.nixos.org/t/new-constraint-between-release-19-09-and-20-03-in-package-qemu/6297

I've tried reverting 2bd296a and it succeeds for me, noting that's the commit that introduced the problem.

To Reproduce
Steps to reproduce the behavior:

1. Launch an EC2 instance
2. Run the following:

$ export NIXOS=/nix/var/nix/profiles/per-user/root/channels/nixos/nixos/
$ nix-build -A amazonImage.x86_64-linux $NIXOS/release.nix

Expected behavior
Qemu would succeed in creating a disk image

Notify maintainers
cc @volth who's the author of the referenced commit. I'm not sure what the right fix is here, so would appreciate some guidance.