vfio/mlx5: Refactor MKEY usage

This patch refactors MKEY usage such as its life cycle will be as of the
migration file instead of allocating/destroying it upon each
SAVE/LOAD command.

This is a preparation step towards the PRE_COPY series where multiple
images will be SAVED/LOADED.

We achieve it by having a new struct named mlx5_vhca_data_buffer which
holds the mkey and its related stuff as of sg_append_table,
allocated_length, etc.

The above fields were taken out from the migration file main struct,
into mlx5_vhca_data_buffer dedicated struct with the proper helpers in
place.

For now we have a single mlx5_vhca_data_buffer per migration file.
However, in coming patches we'll have multiple of them to support
multiple images.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20221206083438.37807-6-yishaih@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>