nixos/lib/qemu-common: Opportunistically use KVM

This will try to use KVM if available and fallback to using TCG if not.
Useful for nested virtualisation of arm on x86.

nixos/lib/qemu-common.nix

@@ -23,8 +23,8 @@ rec {
 
   qemuBinary = qemuPkg: {
     x86_64-linux = "${qemuPkg}/bin/qemu-kvm -cpu max";
-    armv7l-linux = "${qemuPkg}/bin/qemu-system-arm -enable-kvm -machine virt -cpu host";
-    aarch64-linux = "${qemuPkg}/bin/qemu-system-aarch64 -enable-kvm -machine virt,gic-version=host -cpu host";
+    armv7l-linux = "${qemuPkg}/bin/qemu-system-arm -machine virt,accel=kvm:tcg -cpu max";
+    aarch64-linux = "${qemuPkg}/bin/qemu-system-aarch64 -machine virt,gic-version=max,accel=kvm:tcg -cpu max";
     powerpc64le-linux = "${qemuPkg}/bin/qemu-system-ppc64 -machine powernv";
     powerpc64-linux = "${qemuPkg}/bin/qemu-system-ppc64 -machine powernv";
     x86_64-darwin = "${qemuPkg}/bin/qemu-kvm -cpu max";