nixos/qemu-vm: use qemu_kvm

qemu_kvm is only built for one architecture, so it's smaller and takes
MUCH less time to build if it has to be built from source.  And this
module doesn't support running a VM for one architecture from another
architecture, so the one architecture is all we'll need.

nixos/modules/virtualisation/qemu-vm.nix

@@ -560,7 +560,7 @@ in
       package =
         mkOption {
           type = types.package;
-          default = pkgs.qemu;
+          default = pkgs.qemu_kvm;
           example = "pkgs.qemu_test";
           description = "QEMU package to use.";
         };