Merge remote-tracking branch 'remotes/stefanha/tags/block-pull-request' into staging

Merge remote-tracking branch 'remotes/stefanha/tags/block-pull-request' into staging

# gpg: Signature made Tue 05 Sep 2017 14:28:52 BST
# gpg:                using RSA key 0x9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>"
# gpg:                 aka "Stefan Hajnoczi <stefanha@gmail.com>"
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/block-pull-request:
  block: document semantics of bdrv_co_preadv|pwritev
  qemu-iotests: use context managers for resource cleanup in 194
  iotests.py: add FilePath context manager
  qemu.py: make VM() a context manager

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