block: Quiesce old aio context during bdrv_set_aio_context

block: Quiesce old aio context during bdrv_set_aio_context

The fact that the bs->aio_context is changing can confuse the dataplane
iothread, because of the now fine granularity aio context lock.
bdrv_drain should rather be a bdrv_drained_begin/end pair, but since
bs->aio_context is changing, we can just use aio_disable_external and
bdrv_parent_drained_begin.

Reported-by: Ed Swierk <[email protected]>
Signed-off-by: Fam Zheng <[email protected]>
Acked-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>