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>

README.md

@@ -118,11 +118,12 @@ does not run Windows Setup again. `build-windows-base.sh` makes that base image
 installs Windows unattended from the ISO in `install-iso/` (it does everything
 itself: answer file in `windows11/seed/`, QEMU hardware in `windows11/qemu/`; this
 directory has no dependency on any other repository). The answer file boots
-into audit mode (no account and no credentials are needed) and runs
+into audit mode (no account and no credentials are needed), adds the VirtIO drivers
+from the virtio-win ISO (downloaded if missing) to the driver store and runs
 `sysprep /generalize` there as its last step; the script waits for the guest to power
 off and stores the disk read-only. The image is clean and generic, like a cloud
-image: nothing but Windows, with no account, no SSH, no settings, no VirtIO or
-other hardware-specific drivers and no machine identity. Each VM is configured
+image: nothing but Windows and the VirtIO drivers, with no account, no SSH, no settings,
+no other hardware-specific drivers and no machine identity. Each VM is configured
 on its first boot: Windows reads `Unattend.xml` from the root of an attached
 CD-ROM during its specialize and OOBE passes (a VM launcher can mount such a
 seed ISO when it creates a VM disk).