runInLinuxVM: Use QEMU command line that works on other architectures

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

nixos/lib/qemu-flags.nix

···
         else if pkgs.stdenv.isArm || pkgs.stdenv.isAarch64 then "ttyAMA0"
         else throw "Unknown QEMU serial device for system '${pkgs.stdenv.system}'";
 
+  qemuBinary = qemuPkg: {
+    "i686-linux" = "${qemuPkg}/bin/qemu-kvm";
+    "x86_64-linux" = "${qemuPkg}/bin/qemu-kvm -cpu kvm64";
+    "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";
+  }.${pkgs.stdenv.system} or (throw "Unknown QEMU binary for '${pkgs.stdenv.system}'");
 }

nixos/modules/virtualisation/qemu-vm.nix

···
 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 == ""
···
       '')}
 
       # Start QEMU.
-      exec ${qemuKvm} \
+      exec ${qemuBinary qemu} \
           -name ${vmName} \
           -m ${toString config.virtualisation.memorySize} \
           -smp ${toString config.virtualisation.cores} \

pkgs/build-support/vm/default.nix

···
     patches = [ ../../../nixos/modules/virtualisation/azure-qemu-220-no-etc-install.patch ];
   });
 
-  qemuProg = "${qemu}/bin/qemu-kvm";
-
 
   modulesClosure = makeModulesClosure {
     inherit kernel rootModules;
···
 
 
   qemuCommandLinux = ''
-    ${qemuProg} \
-      ${lib.optionalString (pkgs.stdenv.system == "x86_64-linux") "-cpu kvm64"} \
+    ${qemuBinary qemu} \
       -nographic -no-reboot \
       -device virtio-rng-pci \
       -virtfs local,path=${storeDir},security_model=none,mount_tag=store \