migration/multifd: Cleanup outgoing_args in state destroy

migration/multifd: Cleanup outgoing_args in state destroy

outgoing_args is a global cache of socket address to be reused in multifd.
Freeing the cache in per-channel destructor is more or less a hack.  Move
it to multifd_send_cleanup_state() so it only get checked once.  Use a
small helper to do so because it's internal of socket.c.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240222095301.171137-5-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>