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 <eswierk@skyportsystems.com> Signed-off-by: Fam Zheng <famz@redhat.com> Acked-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Kevin Wolf <kwolf@redhat.com>