[Feature] Add iommufd support for VFIO-backed GPU allocation

[johnahull]

Summary

Add iommufd support for VFIO-backed GPU allocation.

Current state

GPU VFIO discovery depends on the legacy PCI iommu_group symlink and CDI exposes /dev/vfio/<group> devices. There is no iommufd or /dev/iommu handling.

Scope

• Detect and use iommufd when available, while preserving a controlled legacy fallback if required.
• Define the CDI/device-node and preparation behavior for iommufd-backed allocations.
• Update driver binding and cleanup paths.
• Add fake-sysfs/unit and end-to-end coverage for iommufd and legacy paths.