runInLinuxVM: Use QEMU command line that works on other architectures

... by moving the existing definition to qemu-flags.nix and reusing
that.

nixos/modules/virtualisation/qemu-vm.nix

@@ -15,12 +15,6 @@ with import ../../lib/qemu-flags.nix { inherit pkgs; };
 let
 
   qemu = config.system.build.qemu or pkgs.qemu_test;
-  qemuKvm = {
-    "i686-linux" = "${qemu}/bin/qemu-kvm";
-    "x86_64-linux" = "${qemu}/bin/qemu-kvm -cpu kvm64";
-    "armv7l-linux" = "${qemu}/bin/qemu-system-arm -enable-kvm -machine virt -cpu host";
-    "aarch64-linux" = "${qemu}/bin/qemu-system-aarch64 -enable-kvm -machine virt,gic-version=host -cpu host";
-  }.${pkgs.stdenv.system};
 
   vmName =
     if config.networking.hostName == ""
@@ -79,7 +73,7 @@ let
       '')}
 
       # Start QEMU.
-      exec ${qemuKvm} \
+      exec ${qemuBinary qemu} \
           -name ${vmName} \
           -m ${toString config.virtualisation.memorySize} \
           -smp ${toString config.virtualisation.cores} \