vfio: selftests: Introduce struct iommu

vfio: selftests: Introduce struct iommu

Introduce struct iommu, which logically represents either a VFIO
container or an iommufd IOAS, depending on which IOMMU mode is used by
the test.

This will be used in a subsequent commit to allow devices to be added to
the same container/iommufd.

Reviewed-by: Alex Mastro <amastro@fb.com>
Tested-by: Alex Mastro <amastro@fb.com>
Reviewed-by: Raghavendra Rao Ananta <rananta@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20251126231733.3302983-6-dmatlack@google.com
Signed-off-by: Alex Williamson <alex@shazbot.org>