Merge remote-tracking branch 'remotes/stsquad/tags/pull-testing-270320-2' into staging

Testing updates:

  - docker updates (various dependencies)
  - travis updates (s390x KVM build)
  - tweak qemu/atomic.h headers in event of clash
  - test/vm updates (NetBSD -> 9.0, FreeBSD -> 12.1)
  - disable MTTCG for mips64/mips64el

# gpg: Signature made Fri 27 Mar 2020 13:45:18 GMT
# gpg:                using RSA key 6685AE99E75167BCAFC8DF35FBD0DB095A9E2A44
# gpg: Good signature from "Alex Bennée (Master Work Key) <alex.bennee@linaro.org>" [full]
# Primary key fingerprint: 6685 AE99 E751 67BC AFC8  DF35 FBD0 DB09 5A9E 2A44

* remotes/stsquad/tags/pull-testing-270320-2:
  .travis.yml: Add a KVM-only s390x job
  tests/docker: Add libepoxy and libudev packages to the Fedora image
  tests/docker: Use Python3 PyYAML in the Fedora image
  tests/docker: Install gcrypt devel package in Debian image
  tests/docker: Keep package list sorted
  configure: disable MTTCG for MIPS guests
  tests/vm: fix basevm config
  tests/vm: update NetBSD to 9.0
  tests/vm: update FreeBSD to 12.1
  tests/vm: move vga setup
  tests/vm: write raw console log
  qemu/atomic.h: add #ifdef guards for stdatomic.h

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