block-migration: Flush requests in blk_mig_cleanup

When cancelling block migration, all in-flight requests of the block
migration must be completed before the data can be freed. This was
visible as failing assertions and segfaults.
Reported-by: Peter Lieven <pl@dlhnet.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block-migration.c

@@ -519,6 +519,8 @@ static void blk_mig_cleanup(void)
     BlkMigDevState *bmds;
     BlkMigBlock *blk;
 
+    bdrv_drain_all();
+
     set_dirty_tracking(0);
 
     while ((bmds = QSIMPLEQ_FIRST(&block_mig_state.bmds_list)) != NULL) {