vfio: selftests: Add documentation

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 introd

vfio: selftests: Add documentation

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>

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