Merge remote-tracking branch 'qmp-unstable/queue/qmp' into staging

Merge remote-tracking branch 'qmp-unstable/queue/qmp' into staging

* qmp-unstable/queue/qmp:
  monitor: Cleanup mon->outbuf on write error
  virtio_rng: replace custom backend API with UserCreatable.complete() callback
  add optional 2nd stage initialization to -object/object-add commands
  vl.c: -object: don't ignore duplicate 'id'
  object_add: consolidate error handling

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