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

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

Pull request

Fixes for QEMU on aarch64 ARM hosts and fdmon-io_uring.

# gpg: Signature made Thu 09 Apr 2020 18:42:01 BST
# gpg:                using RSA key 8695A8BFD3F97CDAAC35775A9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>" [full]
# gpg:                 aka "Stefan Hajnoczi <stefanha@gmail.com>" [full]
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/block-pull-request:
  async: use explicit memory barriers
  aio-wait: delegate polling of main AioContext if BQL not held
  aio-posix: signal-proof fdmon-io_uring

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