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

Pull request

This fix prevents hangs when a drive leaves a throttling group.

# gpg: Signature made Thu 19 Jul 2018 15:28:37 BST
# gpg:                using RSA key 9CA4ABB381AB73C8
# 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:
  throttle-groups: fix hang when group member leaves

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