Currently libvirt requires two qemu derivations: qemu and qemu_kvm which is just a truncated version of qemu (defined as qemu.override { hostCpuOnly = true; }).
This patch exposes an option virtualisation.libvirtd.qemuPackage which allows to choose which package to use:
* pkgs.qemu_kvm if all your guests have the same CPU as host, or
* pkgs.qemu which allows to emulate alien architectures (for example ARMV7L on X86_64), or
* a custom derivation
virtualisation.libvirtd.enableKVM option is vague and could be deprecate in favor of virtualisation.libvirtd.qemuPackage, anyway it does allow to enable/disable kvm.
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