vfio: selftests: Rename struct vfio_iommu_mode to iommu_mode

vfio: selftests: Rename struct vfio_iommu_mode to iommu_mode

Rename struct vfio_iommu_mode to struct iommu_mode since the mode can
include iommufd. This also prepares for splitting out all the IOMMU code
into its own structs/helpers/files which are independent from the
vfio_pci_device code.

No function change intended.

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/20251126231733.3302983-5-dmatlack@google.com
Signed-off-by: Alex Williamson <alex@shazbot.org>