vfio: Dump migration features under debugfs

A debugfs directory was recently added for VFIO devices. Add a new
"features" file under the migration sub-directory to expose which
features the device supports.

Signed-off-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20250918121928.1921871-1-clg@redhat.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>