Add documentation for VFIO selftests in driver-api/vfio-selftests.rst, include it in the driver-api index, and update MAINTAINERS. Since VFIO selftests was introduced it has expanded to include a helper library (libvfio), driver-level abstractions for triggering hardware DMA and interrupts, multiple IOMMU execution modes, dynamic IOVA allocation, and native integration into KVM selftests. Dedicated documentation should be helpful to guide kernel developers on writing tests, setting up devices, running tests, and understanding the helper library. Assisted-by: Gemini:3.1-Pro Signed-off-by: David Matlack <dmatlack@google.com> Link: https://lore.kernel.org/r/20260903183814.2111748-1-dmatlack@google.com Signed-off-by: Alex Williamson <alex@shazbot.org>