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

Block layer patches:

- Fix internal snapshots with typical -blockdev setups
- iotests: Require Python 3.6 or later

# gpg: Signature made Fri 04 Oct 2019 10:59:21 BST
# 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: Remove Python 2 compatibility code
  iotests: Require Python 3.6 or later
  iotests: Test internal snapshots with -blockdev
  block/snapshot: Restrict set of snapshot nodes

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