block: all I/O should be completed before removing throttle timers.

block: all I/O should be completed before removing throttle timers.

In blk_remove_bs, all I/O should be completed before removing throttle
timers. If there has inflight I/O, removing throttle timers here will
cause the inflight I/O never return.
This patch add bdrv_drained_begin before throttle_timers_detach_aio_context
to let all I/O completed before removing throttle timers.

[Moved declaration of bs as suggested by Alberto Garcia
<berto@igalia.com>.
--Stefan]

Signed-off-by: Zhengui <lizhengui@huawei.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 1508564040-120700-1-git-send-email-lizhengui@huawei.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>