vfio: selftests: Make iommufd the default iommu_mode

vfio: selftests: Make iommufd the default iommu_mode

Now that VFIO selftests support iommufd, make it the default mode.
IOMMUFD is the successor to VFIO_TYPE1{,v2}_IOMMU and all new features
are being added there, so it's a slightly better fit as the default
mode.

Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20250822212518.4156428-30-dmatlack@google.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>