Reported to me off-list by Laurent Vivier, who found the
problem while working on https://bugzilla.redhat.com/1433193
Broken since 2.7, but the fix is a one-liner (pointing out my
embarrassing mistake of mis-converting a pre-decrement operator);
as a bug fix, it still qualifies for 2.9 in spite of hard freeze,
on the other hand, as the regression was not introduced in 2.9,
I also understand if it is postponed.

v1 was here:
https://lists.gnu.org/archive/html/qemu-devel/2017-03/msg04072.html

Since then:
- fix 'make check' failure, where we had accidentally been relying
on ignoring bogus keys in user_creatable_add_opts() (new patch 2) [Markus]
- commit message and one-word comment improvement, explaining why
test doesn't fail even though it flags problem (patch 1) [Laurent]
- add Reviewed/Tested-by as appropriate

Eric Blake (3):
  tests: Expose regression in QemuOpts visitor
  qom: Avoid unvisited 'id'/'qom-type' in user_creatable_add_opts
  qapi: Fix QemuOpts visitor regression on unvisited input

 qapi/opts-visitor.c       |  6 +++---
 qom/object_interfaces.c   |  7 ++++---
 tests/test-opts-visitor.c | 29 ++++++++++++++++++++++++++---
 3 files changed, 33 insertions(+), 9 deletions(-)

-- 
2.9.3