x86: Track message based interrupt cleanup functions in sender state.

x86: Track message based interrupt cleanup functions in sender state.

This makes sure the completion function follows the packet, and allows
multiple packets to be in flight at once without the functions
overwritting each other.

Change-Id: Ic49c7b646d56b32c0453931942ee22ae07828bb6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26163
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Ayaz Akram <yazakram@ucdavis.edu>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>