vfio/mlx5: Improve the source side flow upon pre_copy

Improve the source side flow upon pre_copy as of below.

- Prepare the stop_copy buffers as part of moving to pre_copy.
- Send to the target a record that includes the expected
  stop_copy size to let it optimize its stop_copy flow as well.

As for sending the target this new record type (i.e.
MLX5_MIGF_HEADER_TAG_STOP_COPY_SIZE) we split the current 64 header
flags bits into 32 flags bits and another 32 tag bits, each record may
have a tag and a flag whether it's optional or mandatory. Optional
records will be ignored in the target.

The above reduces the downtime upon stop_copy as the relevant data stuff
is prepared ahead as part of pre_copy.

Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20230124144955.139901-3-yishaih@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>