Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-12-09' into staging

bitmap fix for 4.2-rc5

- Fix a regression that broke bitmap deletion without a transaction,
and causes a crash with transaction (only transaction is new to 4.2),
when a qcow2 file contains persistent bitmaps from prior shutdown

# gpg: Signature made Mon 09 Dec 2019 15:28:19 GMT
# gpg:                using RSA key 71C2CC22B1C4602927D2F3AAA7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>" [full]
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>" [full]
# gpg:                 aka "[jpeg image of size 6874]" [full]
# Primary key fingerprint: 71C2 CC22 B1C4 6029 27D2  F3AA A7A1 6B4A 2527 436A

* remotes/ericb/tags/pull-nbd-2019-12-09:
  block/qcow2-bitmap: fix crash bug in qcow2_co_remove_persistent_dirty_bitmap

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>