Install the VirtIO drivers into the base image in audit mode

Add Install-VirtIO-Drivers.ps1, run from Autounattend.xml before sysprep, and
attach the virtio-win ISO (downloaded if missing) during the build.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

windows11/seed/Install-VirtIO-Drivers.ps1

@@ -0,0 +1,38 @@
+# Runs in audit mode, before sysprep: adds the VirtIO drivers from the virtio-win
+# ISO to the driver store. The devices are not present during the installation
+# (it uses ramfb and e1000e), so pnputil only stages the packages; they survive
+# /generalize, and Windows binds them on the first start with virtio devices.
+$ErrorActionPreference = 'Stop'
+
+$arch = if ([Runtime.InteropServices.RuntimeInformation]::OSArchitecture -eq 'Arm64') { 'ARM64' } else { 'amd64' }
+$roots = (Get-PSDrive -PSProvider FileSystem).Root
+$drivers = @(
+    @{ Folder = 'NetKVM'; Inf = 'netkvm.inf'; Required = $true },
+    @{ Folder = 'viogpudo'; Inf = 'viogpudo.inf'; Required = $true },
+    @{ Folder = 'vioserial'; Inf = 'vioser.inf' },
+    @{ Folder = 'Balloon'; Inf = 'balloon.inf' },
+    @{ Folder = 'viorng'; Inf = 'viorng.inf' },
+    @{ Folder = 'vioinput'; Inf = 'vioinput.inf' }
+)
+
+$installed = 0
+foreach ($driver in $drivers) {
+    $inf = $roots |
+        ForEach-Object { Join-Path $_ "$($driver.Folder)\w11\$arch\$($driver.Inf)" } |
+        Where-Object { Test-Path -LiteralPath $_ } |
+        Select-Object -First 1
+    if (-not $inf) {
+        if ($driver.Required) {
+            throw "The $arch $($driver.Folder) driver was not found on the attached media."
+        }
+        continue
+    }
+
+    & pnputil.exe /add-driver $inf /install | Out-Host
+    # 3010 asks for a restart; 259 means that no device uses the driver.
+    if ($LASTEXITCODE -notin 0, 259, 3010) {
+        throw "pnputil failed to install $inf (exit code $LASTEXITCODE)."
+    }
+    $installed++
+}
+Write-Host "Added $installed VirtIO $arch driver packages."