Merge remote-tracking branch 'remotes/famz/tags/block-and-testing-pull-request' into...

Block and testing patches for 3.1

- aio fixes by me
- nvme fixes by Paolo and me
- test improvements by Peter, Phil and me

# gpg: Signature made Wed 15 Aug 2018 04:11:43 BST
# gpg:                using RSA key CA35624C6A9171C6
# gpg: Good signature from "Fam Zheng <famz@redhat.com>"
# Primary key fingerprint: 5003 7CB7 9706 0F76 F021  AD56 CA35 624C 6A91 71C6

* remotes/famz/tags/block-and-testing-pull-request:
  aio-posix: Improve comment around marking node deleted
  tests/vm: Add vm-build-all/vm-clean-all in help text
  tests/vm: Use make's --output-sync option
  tests/vm: Bump guest RAM up from 2G to 4G
  tests/vm: Propagate V=1 down into the make inside the VM
  tests/vm: Pass the jobs parallelism setting to 'make check'
  tests: vm: Add vm-clean-all
  tests: Add centos VM testing
  tests: Allow overriding archive path with SRC_ARCHIVE
  tests: Add an option for snapshot (default: off)
  docker: Install more packages in centos7
  aio: Do aio_notify_accept only during blocking aio_poll
  aio-posix: Don't count ctx->notifier as progress when polling
  nvme: simplify plug/unplug
  nvme: Fix nvme_init error handling
  tests/vm: Add flex and bison to the vm image
  tests/vm: Only use -cpu 'host' if KVM is available

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