migration/multifd: Cleanup multifd_save_cleanup()

migration/multifd: Cleanup multifd_save_cleanup()

Shrink the function by moving relevant works into helpers: move the thread
join()s into multifd_send_terminate_threads(), then create two more helpers
to cover channel/state cleanups.

Add a TODO entry for the thread terminate process because p->running is
still buggy.  We need to fix it at some point but not yet covered.

Suggested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240202102857.110210-20-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>