Documentation: add debugfs description for vfio

Add an debugfs document description file to help users understand
how to use the accelerator live migration driver's debugfs.

Signed-off-by: Longfang Liu <liulongfang@huawei.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20231106072225.28577-3-liulongfang@huawei.com


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