When ending a drained section, bdrv_do_drained_end() currently first
decrements the quiesce_counter, and only then actually ends the drain.

The bdrv_drain_invoke(bs, false) call may cause graph changes.  Say the
graph change involves replacing an existing BB's ("blk") BDS
(blk_bs(blk)) by @bs.  Let us introducing the following values:
- bs_oqc = old_quiesce_counter
  (so bs->quiesce_counter == bs_oqc - 1)
- obs_qc = blk_bs(blk)->quiesce_counter (before bdrv_drain_invoke())

Let us assume there is no blk_pread_unthrottled() involved, so
blk->quiesce_counter == obs_qc (before bdrv_drain_invoke()).

Now replacing blk_bs(blk) by @bs will reduce blk->quiesce_counter by
obs_qc (making it 0) and increase it by bs_oqc-1 (making it bs_oqc-1).

bdrv_drain_invoke() returns and we invoke bdrv_parent_drained_end().
This will decrement blk->quiesce_counter by one, so it would be -1 --
were there not an assertion against that in blk_root_drained_end().

We therefore have to keep the quiesce_counter up at least until
bdrv_drain_invoke() returns, so that bdrv_parent_drained_end() does the
right thing for the parents @bs got during bdrv_drain_invoke().

But let us delay it even further, namely until bdrv_parent_drained_end()
returns, because then it mirrors bdrv_do_drained_begin(): There, we
first increment the quiesce_counter, then begin draining the parents,
and then call bdrv_drain_invoke().  It makes sense to let
bdrv_do_drained_end() unravel this exactly in reverse.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>