nixos/qemu: Deduplicate QEMU serialDevice into qemu-flags.nix

nixos/modules/virtualisation/qemu-vm.nix

@@ -10,6 +10,7 @@
 { config, lib, pkgs, ... }:
 
 with lib;
+with import ../../lib/qemu-flags.nix { inherit pkgs; };
 
 let
 
@@ -21,11 +22,6 @@ let
     "aarch64-linux" = "${qemu}/bin/qemu-system-aarch64 -enable-kvm -machine virt,gic-version=host -cpu host";
   }.${pkgs.stdenv.system};
 
-  # FIXME: figure out a common place for this instead of copy pasting
-  serialDevice = if pkgs.stdenv.isi686 || pkgs.stdenv.isx86_64 then "ttyS0"
-        else if pkgs.stdenv.isArm || pkgs.stdenv.isAarch64 then "ttyAMA0"
-        else throw "Unknown QEMU serial device for system '${pkgs.stdenv.system}'";
-
   vmName =
     if config.networking.hostName == ""
     then "noname"
@@ -34,7 +30,7 @@ let
   cfg = config.virtualisation;
 
   qemuGraphics = if cfg.graphics then "" else "-nographic";
-  kernelConsole = if cfg.graphics then "" else "console=${serialDevice}";
+  kernelConsole = if cfg.graphics then "" else "console=${qemuSerialDevice}";
   ttys = [ "tty1" "tty2" "tty3" "tty4" "tty5" "tty6" ];
 
   # Shell script to start the VM.