Merge remote-tracking branch 'remotes/cleber/tags/python-next-pull-request' into staging

Python and tests (mostly acceptance) patches 2020-03-17

# gpg: Signature made Wed 18 Mar 2020 00:16:03 GMT
# gpg:                using RSA key 7ABB96EB8B46B94D5E0FE9BB657E8D33A5F209F3
# gpg: Good signature from "Cleber Rosa <crosa@redhat.com>" [marginal]
# gpg: WARNING: This key is not certified with sufficiently trusted signatures!
# gpg:          It is not certain that the signature belongs to the owner.
# Primary key fingerprint: 7ABB 96EB 8B46 B94D 5E0F  E9BB 657E 8D33 A5F2 09F3

* remotes/cleber/tags/python-next-pull-request:
  tests/docker: make "buildah bud" output similar to "docker build"
  tests/docker: add CentOS 8 Dockerfile
  Acceptance tests: add make targets to download images
  Acceptance test: add "boot_linux" tests
  Acceptance tests: introduce BUILD_DIR and SOURCE_DIR
  python/qemu/qmp.py: QMP debug with VM label

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