nixos/qemu-vm: set permissions for tmpfs root

World-writable fs root breaks ssh key authentication (and likely other
programs).

nixos/modules/virtualisation/qemu-vm.nix

@@ -1267,6 +1267,7 @@ in
               {
                 device = "tmpfs";
                 fsType = "tmpfs";
+                options = [ "mode=755" ];
               }
             else
               {