vfio: Define uAPI for re-init initial bytes during the PRE_COPY phase

As currently defined, initial_bytes is monotonically decreasing and
precedes dirty_bytes when reading from the saving file descriptor.
The transition from initial_bytes to dirty_bytes is unidirectional and
irreversible.

The initial_bytes are considered as critical data that is highly
recommended to be transferred to the target as part of PRE_COPY, without
this data, the PRE_COPY phase would be ineffective.

We come to solve the case when a new chunk of critical data is
introduced during the PRE_COPY phase and the driver would like to report
an entirely new value for the initial_bytes.

For that, we extend the VFIO_MIG_GET_PRECOPY_INFO ioctl with an output
flag named VFIO_PRECOPY_INFO_REINIT to allow drivers reporting a new
initial_bytes value during the PRE_COPY phase.

Currently, existing VFIO_MIG_GET_PRECOPY_INFO implementations don't
assign info.flags before copy_to_user(), this effectively echoes
userspace-provided flags back as output, preventing the field from being
used to report new reliable data from the drivers.

Reliable use of the new VFIO_PRECOPY_INFO_REINIT flag requires userspace
to explicitly opt in by enabling the
VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2 device feature.

When the caller opts in, the driver may report an entirely new
value for initial_bytes. It may be larger, it may be smaller, it may
include the previous unread initial_bytes, it may discard the previous
unread initial_bytes, up to the driver logic and state.
The presence of the VFIO_PRECOPY_INFO_REINIT output flag set by the
driver indicates that new initial data is present on the stream.

Once the caller sees this flag, the initial_bytes value should be
re-evaluated relative to the readiness state for transition to
STOP_COPY.

Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20260317161753.18964-2-yishaih@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>