Document interfaces used for VFIO device migration. Added flow
of state changes during live migration with VFIO device.

Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Co-developed-by: Kirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: Tarun Gupta <targupta@nvidia.com>
Message-Id: <20210418122251.88809-1-targupta@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>