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

Acceptance Tests and Python libs improvements

Along with the Acceptance Tests and Python libs improvements, a
improvement to the diff generation for Python code.

# gpg: Signature made Tue 16 Feb 2021 04:55:45 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-gitlab/tags/python-next-pull-request:
  Acceptance Tests: set up existing ssh keys by default
  Acceptance Tests: fix population of public key in cloudinit image
  Acceptance Tests: introduce method for requiring an accelerator
  Acceptance Tests: introduce LinuxTest base class
  maint: Tell git that *.py files should use python diff hunks
  tests/acceptance/virtio-gpu.py: preserve virtio-user-gpu log
  Python: close the log file kept by QEMUMachine before reading it
  virtiofs_submounts.py test: Note on vmlinuz param
  Acceptance Tests: bump Avocado version requirement to 85.0

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