vfio: selftests: fix map limit tests to use last available iova

vfio: selftests: fix map limit tests to use last available iova

Use the newly available vfio_pci_iova_ranges() to determine the last
legal IOVA, and use this as the basis for vfio_dma_map_limit_test tests.

Fixes: de8d1f2fd5a5 ("vfio: selftests: add end of address space DMA map/unmap tests")
Reviewed-by: David Matlack <dmatlack@google.com>
Tested-by: David Matlack <dmatlack@google.com>
Signed-off-by: Alex Mastro <amastro@fb.com>
Link: https://lore.kernel.org/r/20251111-iova-ranges-v3-2-7960244642c5@fb.com
Signed-off-by: Alex Williamson <alex@shazbot.org>