vfio: selftests: Encapsulate IOMMU mode

vfio: selftests: Encapsulate IOMMU mode

Encapsulate the "IOMMU mode" a test should use behind a new struct.
In the future this will be used to support other types of IOMMUs besides
VFIO_TYPE1_IOMMU, and allow users to select the mode on the command
line.

No functional change intended.

Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20250822212518.4156428-25-dmatlack@google.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>