blockdev: no need to drain in qmp_block_commit

Draining is not necessary, I/O can happen as soon as the
commit coroutine yields.  Draining can be necessary before
reopening the file for read/write, or while modifying the
backing file chain, but that is done separately in
bdrv_reopen_multiple or bdrv_close; this particular
bdrv_drain_all does nothing for that.

Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-id: [email protected]
Signed-off-by: Jeff Cody <[email protected]>

blockdev.c

@@ -2380,9 +2380,6 @@ void qmp_block_commit(const char *device,
     aio_context = bdrv_get_aio_context(bs);
     aio_context_acquire(aio_context);
 
-    /* drain all i/o before commits */
-    bdrv_drain_all();
-
     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_COMMIT_SOURCE, errp)) {
         goto out;
     }