Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer patches:

- Fix HMP savevm with -blockdev
- Minor iotests improvements

# gpg: Signature made Mon 18 Nov 2019 16:51:56 GMT
# gpg:                using RSA key 7F09B272C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>" [full]
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74  56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream:
  iotests: Test multiple blockdev-snapshot calls
  block: Remove 'backing': null from bs->{explicit_,}options
  iotests: Fix "no qualified output" error path
  qemu-iotests/iotests.py: improve assert_qmp message

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