vfio: selftests: Add iommufd_compat_type1{,v2} modes

vfio: selftests: Add iommufd_compat_type1{,v2} modes

Add new IOMMU modes for using iommufd in compatibility mode with
VFIO_TYPE1_IOMMU and VFIO_TYPE1v2_IOMMU.

In these modes, VFIO selftests will open /dev/iommu and treats it as a
container FD (as if it had opened /dev/vfio/vfio) and the kernel
translates the container ioctls to iommufd calls transparently.

Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20250822212518.4156428-28-dmatlack@google.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>