migration/block: Avoid invoking blk_drain too frequently

Increase bmds->cur_dirty after submit io, so reduce the frequency
involve into blk_drain, and improve the performance obviously
when block migration.

The performance test result of this patch:

During the block dirty save phase, this patch improve guest os IOPS
from 4.0K to 9.5K. and improve the migration speed from
505856 rsec/s to 855756 rsec/s.
Signed-off-by: NLidong Chen <jemmy858585@gmail.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

migration/block.c

@@ -576,6 +576,9 @@ static int mig_save_device_dirty(QEMUFile *f, BlkMigDevState *bmds,
             }
 
             bdrv_reset_dirty_bitmap(bmds->dirty_bitmap, sector, nr_sectors);
+            sector += nr_sectors;
+            bmds->cur_dirty = sector;
+
             break;
         }
         sector += BDRV_SECTORS_PER_DIRTY_CHUNK;