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/Autounattend.xml

@@ -131,6 +131,11 @@
         </RunSynchronousCommand>
         <RunSynchronousCommand wcm:action="add">
           <Order>4</Order>
+          <Description>Add the VirtIO drivers from the virtio-win ISO to the driver store</Description>
+          <Path>powershell.exe -NoProfile -ExecutionPolicy Bypass -Command "&amp; ('{0}:\Install-VirtIO-Drivers.ps1' -f (Get-Volume -FileSystemLabel ANSWER).DriveLetter); exit $LASTEXITCODE"</Path>
+        </RunSynchronousCommand>
+        <RunSynchronousCommand wcm:action="add">
+          <Order>5</Order>
           <Description>Generalize the image and power off</Description>
           <Path>C:\Windows\System32\Sysprep\sysprep.exe /generalize /oobe /shutdown /quiet</Path>
         </RunSynchronousCommand>