vfio: Add support for VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2

vfio: Add support for VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2

Currently, existing VFIO_MIG_GET_PRECOPY_INFO implementations don't
assign info.flags before copy_to_user().

Because they copy the struct in from userspace first, this effectively
echoes userspace-provided flags back as output, preventing the field
from being used to report new reliable data from the drivers.

Add support for a new device feature named
VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2.

On SET, enables the v2 pre_copy_info behaviour, where the
vfio_precopy_info.flags is a valid output field.

Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20260317161753.18964-3-yishaih@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>