migration: Deprecate query-migrationthreads command

Per previous discussion [1,2], this patch deprecates query-migrationthreads
command.

To summarize, the major reason of the deprecation is due to no sensible way
to consume the API properly:

  (1) The reported list of threads are incomplete (ignoring destination
      threads and non-multifd threads).

  (2) For CPU pinning, there's no way to properly pin the threads with
      the API if the threads will start running right away after migration
      threads can be queried, so the threads will always run on the default
      cores for a short window.

  (3) For VM debugging, one can use "-name $VM,debug-threads=on" instead,
      which will provide proper names for all migration threads.

[1] https://lore.kernel.org/r/20240930195837.825728-1-peterx@redhat.com
[2] https://lore.kernel.org/r/20241011153417.516715-1-peterx@redhat.com

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Acked-by: Markus Armbruster <armbru@redhat.com>
Link: https://lore.kernel.org/r/20241022194501.1022443-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

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