migration: Use warn_reportf_err() where appropriate

Replace

    warn_report("...: %s", ..., error_get_pretty(err));

by

    warn_reportf_err(err, "...: ", ...);

Prior art: commit 5217f1887a (error: Use error_reportf_err() where
appropriate).

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20251115083500.2753895-3-armbru@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/multifd.c

@@ -464,8 +464,8 @@ static void migration_ioc_shutdown_gracefully(QIOChannel *ioc)
          */
         migration_tls_channel_end(ioc, &local_err);
         if (local_err) {
-            warn_report("Failed to gracefully terminate TLS connection: %s",
-                        error_get_pretty(local_err));
+            warn_reportf_err(local_err,
+                        "Failed to gracefully terminate TLS connection: ");
         }
     }