Add an `iommufd` flag to existing VFIO integration tests. When false,
tests use the legacy vfio container/group backend (existing behavior).
When true, tests use vfio cdev with iommufd and vfio_p2p_dma=off.
vfio_p2p_dma=off is required because the VFIO test runner uses a stock
Ubuntu 24.04 kernel (v6.8) which does not support mapping device MMIO
pages (VM_PFNMAP) through iommufd, causing IOMMU_IOAS_MAP to fail with
-EFAULT on MMIO BAR regions.
Signed-off-by: Bo Chen <bchen@crusoe.ai>