Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-01-14' into staging

nbd patches for 2019-01-14

Promote bitmap/NBD interfaces to stable for use in incremental
backups. Add 'qemu-nbd --bitmap'.

- John Snow: 0/11 bitmaps: remove x- prefix from QMP api
- Philippe Mathieu-Daudé: qemu-nbd: Rename 'exp' variable clashing with math::exp() symbol
- Eric Blake: 0/8 Promote x-nbd-server-add-bitmap to stable

# gpg: Signature made Mon 14 Jan 2019 16:13:45 GMT
# gpg:                using RSA key A7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>"
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>"
# gpg:                 aka "[jpeg image of size 6874]"
# Primary key fingerprint: 71C2 CC22 B1C4 6029 27D2  F3AA A7A1 6B4A 2527 436A

* remotes/ericb/tags/pull-nbd-2019-01-14:
  qemu-nbd: Add --bitmap=NAME option
  nbd: Merge nbd_export_bitmap into nbd_export_new
  nbd: Remove x-nbd-server-add-bitmap
  nbd: Allow bitmap export during QMP nbd-server-add
  nbd: Merge nbd_export_set_name into nbd_export_new
  nbd: Only require disabled bitmap for read-only exports
  nbd: Forbid nbd-server-stop when server is not running
  nbd: Add some error case testing to iotests 223
  qemu-nbd: Rename 'exp' variable clashing with math::exp() symbol
  iotests: add iotest 236 for testing bitmap merge
  iotests: implement pretty-print for log and qmp_log
  iotests: change qmp_log filters to expect QMP objects only
  iotests: remove default filters from qmp_log
  iotests: add qmp recursive sorting function
  iotests: add filter_generated_node_ids
  iotests.py: don't abort if IMGKEYSECRET is undefined
  block: remove 'x' prefix from experimental bitmap APIs
  blockdev: n-ary bitmap merge
  block/dirty-bitmap: remove assertion from restore
  blockdev: abort transactions in reverse order

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