vfio: selftests: update DMA map/unmap helpers to support more test kinds

Add __vfio_pci_dma_*() helpers which return -errno from the underlying
ioctls.

Add __vfio_pci_dma_unmap_all() to test more unmapping code paths. Add an
out unmapped arg to report the unmapped byte size.

The existing vfio_pci_dma_*() functions, which are intended for
happy-path usage (assert on failure) are now thin wrappers on top of the
double-underscore helpers.

Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Alex Mastro <amastro@fb.com>
Link: https://lore.kernel.org/r/20251028-fix-unmap-v6-4-2542b96bcc8e@fb.com


Signed-off-by: Alex Williamson <alex@shazbot.org>