vfio: selftests: Rename struct vfio_dma_region to dma_region

vfio: selftests: Rename struct vfio_dma_region to dma_region

Rename struct vfio_dma_region to dma_region. This is in preparation for
separating the VFIO PCI device library code from the IOMMU library code.
This name change also better reflects the fact that DMA mappings can be
managed by either VFIO or IOMMUFD. i.e. the "vfio_" prefix is
misleading.

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/20251126231733.3302983-11-dmatlack@google.com
Signed-off-by: Alex Williamson <alex@shazbot.org>