vfio: selftests: Move vfio dma mapping test to their own file

vfio: selftests: Move vfio dma mapping test to their own file

Move the dma_map_unmap test from vfio_pci_device_test to a new test:
vfio_dma_mapping_test. We are going to add more complex dma mapping
tests, so it makes sense to separate this from the vfio pci device
test which is more of a sanity check for vfio pci functionality.

Signed-off-by: Josh Hilke <jrhilke@google.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20250822212518.4156428-6-dmatlack@google.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>