migration: Remember total dirty bytes in mig_stats

Introduce this new counter to remember the total dirty bytes for the whole system. It will be used for query-migrate command to fetch system-wise remaining data. A prior attempt was made to not use this counter but query directly from all the modules in a QMP handler, but it exposed some complexity not only on migration state machine race conditions (where the query may be invoked anytime of the state machine), or on locking implications (where some of the query hooks may take BQL, which is illegal at least in a QMP handler). For more information, see: https://lore.kernel.org/r/aeZMtxqrKWAMKzdN@x1.local This oneliner will resolve everything, except that it is not as accurate. The hope is it is a worthwhile trade-off solution, after knowing above challenges. Now, there is one more reason we should make each invocation of save_live_iterate() to be lightweight, because this counter will only get updated once for each loop over all save_live_iterate() hooks when present. But that's always the goal. Reviewed-by: Juraj Marcin <jmarcin@redhat.com> Link: https://lore.kernel.org/r/20260421202110.306051-14-peterx@redhat.com Signed-off-by: Peter Xu <peterx@redhat.com>