vfio/mlx5: Introduce vfio precopy ioctl implementation

vfio precopy ioctl returns an estimation of data available for
transferring from the device.

Whenever a user is using VFIO_MIG_GET_PRECOPY_INFO, track the current
state of the device, and if needed, append the dirty data to the
transfer FD data. This is done by saving a middle state.

As mlx5 runs the SAVE command asynchronously, make sure to query for
incremental data only once there is no active save command.
Running both in parallel, might end-up with a failure in the incremental
query command on un-tracked vhca.

Also, a middle state will be saved only after the previous state has
finished its SAVE command and has been fully transferred, this prevents
endless use resources.

Co-developed-by: Shay Drory <[email protected]>
Signed-off-by: Shay Drory <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Signed-off-by: Yishai Hadas <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alex Williamson <[email protected]>

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