migration: calculate expected_downtime with ram_bytes_remaining()

expected_downtime value is not accurate with dirty_pages_rate * page_size,
using ram_bytes_remaining() would yeild it resonable.

consider to read the remaining ram just after having updated the dirty
pages count later migration_bitmap_sync_range() in migration_bitmap_sync()
and reuse the `remaining` field in ram_counters to hold ram_bytes_remaining()
for calculating expected_downtime.

Reported-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20180612085009.17594-2-bala24@linux.vnet.ibm.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/migration.c

@@ -2746,8 +2746,7 @@ static void migration_update_counters(MigrationState *s,
      * recalculate. 10000 is a small enough number for our purposes
      */
     if (ram_counters.dirty_pages_rate && transferred > 10000) {
-        s->expected_downtime = ram_counters.dirty_pages_rate *
-            qemu_target_page_size() / bandwidth;
+        s->expected_downtime = ram_counters.remaining / bandwidth;
     }
 
     qemu_file_reset_rate_limit(s->to_dst_file);

migration/ram.c

@@ -1180,6 +1180,7 @@ static void migration_bitmap_sync(RAMState *rs)
     RAMBLOCK_FOREACH_MIGRATABLE(block) {
         migration_bitmap_sync_range(rs, block, 0, block->used_length);
     }
+    ram_counters.remaining = ram_bytes_remaining();
     rcu_read_unlock();
     qemu_mutex_unlock(&rs->bitmap_mutex);