qemu: Add audio support to virtual machines

This fixes an issue where virtual machines would previously use dummy
output. More specifically, pulseaudio is used since pipewire supports
it, and intel hd audio (hda) is used since it's generally regarded as
one of the better options based on information available online.

Note that -audio is used instead of the outdated -audiodev and -soundhw
options. This setup also avoids the choppy audio observed with other
solutions like alsa.

References:
- https://www.qemu.org/docs/master/system/qemu-manpage.html
- https://github.com/NixOS/nixpkgs/issues/175378
- https://superuser.com/questions/1337570
- https://en.wikipedia.org/wiki/Intel_High_Definition_Audio
- https://gitlab.com/qemu-project/qemu/-/issues/1519
- https://lists.nongnu.org/archive/html/qemu-discuss/2020-04/msg00089.html

src/main.nix

@@ -236,7 +236,7 @@ let VARIABLES = import ./variables.nix; in {
       cores = 4;
     };
 
-    virtualisation.qemu.options = [ "-device virtio-vga-gl" "-display sdl,gl=on,show-cursor=off" "-full-screen" ];
+    virtualisation.qemu.options = [ "-device virtio-vga-gl" "-display sdl,gl=on,show-cursor=off" "-full-screen" "-audio pa,model=hda" ];
 
     environment.sessionVariables = {
       WLR_NO_HARDWARE_CURSORS = "1";