blockdev: no need to drain in qmp_block_commit

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: 1432822903[email protected]
Signed-off-by: Jeff Cody <[email protected]>