Add QEMU Guest Agent installation from VirtIO ISO

[kallioli]

This PR adds the ability to install the QEMU Guest Agent directly from a VirtIO ISO file instead of always downloading from the internet.

⚠️ Note: This PR depends on #413 (QEMU GA checksum verification) and should be merged after it.

New Features

• New source parameter with three options:
  • web: Download from internet (default, backward compatible)
  • iso: Extract from VirtIO ISO only
  • auto: Try ISO first, fallback to web
• New Copy-QemuGuestAgentFromISO function for ISO extraction
• Automatic architecture detection (x86_64/i386)
• Comprehensive search with fallback paths and recursive search

Configuration Examples

Offline Mode

[drivers]
virtio_iso_path=/path/to/virtio-win.iso
 
[custom]
install_qemu_ga=True
 
[virtio_qemu_guest_agent]
source=iso
Automatic Mode (Recommended)
ini
[drivers]
virtio_iso_path=/path/to/virtio-win.iso
[custom]
install_qemu_ga=True
[virtio_qemu_guest_agent]
source=auto

Benefits

• Offline environments: Support for air-gapped systems
• Faster builds: No network download required
• Version consistency: Match guest agent with VirtIO drivers version
• Bandwidth savings: Reuse ISO for multiple image builds
• Flexibility: auto mode works both online and offline

Technical Details

• Mounts ISO using Windows VirtualDisk API
• Searches multiple possible paths for the MSI file
• Includes recursive search as fallback
• Safe ISO dismount with cleanup
• Backward Compatibility
• All existing configurations continue to work without modification. The default source=web maintains current behavior.

Files Modified

Config.psm1
WinImageBuilder.psm1
Examples/windows-image-config-example.ini
README.md