feat: Add graphics support for vfkit hypervisor

Enables vfkit to use the --gui flag and virtio-gpu devices when
microvm.graphics.enable is set to true, matching the behavior of
other hypervisors like qemu and cloud-hypervisor.

Changes:
- Add virtio-gpu, virtio-input keyboard and pointing devices when
  graphics.enable is true
- Add --gui flag to vfkit command line when graphics are enabled
- Replace virtio-serial with GUI devices in graphics mode
- Update README to document graphics support on macOS with vfkit

Author: luizribeiro

README.md

@@ -100,12 +100,15 @@ nix run microvm#vm
 Check `networkctl status virbr0` for the DHCP leases of the nested
 MicroVMs. They listen for ssh with an empty root password.
 
-### Experimental: run graphical applications in cloud-hypervisor with Wayland forwarding
+### Experimental: run graphical applications with graphics support
 
+On Linux with cloud-hypervisor and Wayland forwarding:
 ```shell
 nix run microvm#graphics neverball
 ```
 
+On macOS with vfkit, enable graphics with `microvm.graphics.enable = true`.
+
 ## Commercial support
 
 Accelerate your operations and secure your infrastructure with support from a

lib/runners/vfkit.nix

@@ -15,7 +15,7 @@ let
   inherit (microvmConfig)
     hostName vcpu mem user interfaces volumes shares socket
     storeOnDisk kernel initrdPath storeDisk kernelParams
-    balloon devices credentialFiles vsock;
+    balloon devices credentialFiles vsock graphics;
 
   inherit (microvmConfig.vfkit) extraArgs logLevel;
 
@@ -24,7 +24,8 @@ let
   # vfkit requires uncompressed kernel
   kernelPath = "${kernel.out}/${pkgs.stdenv.hostPlatform.linux-kernel.target}";
 
-  kernelCmdLine = "console=hvc0 reboot=t panic=-1 ${toString kernelParams}";
+  kernelConsole = if graphics.enable then "tty0" else "hvc0";
+  kernelCmdLine = "console=${kernelConsole} reboot=t panic=-1 ${toString kernelParams}";
 
   bootloaderArgs = [
     "--bootloader"
@@ -34,7 +35,13 @@ let
   deviceArgs =
     [ "--device" "virtio-rng" ]
     ++
-    [ "--device" "virtio-serial,stdio" ]
+    (if graphics.enable then [
+      "--device" "virtio-gpu"
+      "--device" "virtio-input,keyboard"
+      "--device" "virtio-input,pointing"
+    ] else [
+      "--device" "virtio-serial,stdio"
+    ])
     ++
     (builtins.concatMap ({ image, ... }: [
       "--device" "virtio-blk,path=${image}"
@@ -72,6 +79,9 @@ let
   ++ lib.optionals (logLevel != null) [
     "--log-level" logLevel
   ]
+  ++ lib.optionals graphics.enable [
+    "--gui"
+  ]
   ++ bootloaderArgs
   ++ deviceArgs
   ++ extraArgs;