Merge remote-tracking branch 'remotes/aperard/tags/pull-xen-20190624' into staging

Xen queue

* Fix build
* xen-block: support feature-large-sector-size
* xen-block: Support IOThread polling for PV shared rings
* Avoid usage of a VLA
* Cleanup Xen headers usage

# gpg: Signature made Mon 24 Jun 2019 16:30:32 BST
# gpg:                using RSA key F80C006308E22CFD8A92E7980CF5572FD7FB55AF
# gpg:                issuer "anthony.perard@citrix.com"
# gpg: Good signature from "Anthony PERARD <anthony.perard@gmail.com>" [marginal]
# gpg:                 aka "Anthony PERARD <anthony.perard@citrix.com>" [marginal]
# gpg: WARNING: This key is not certified with sufficiently trusted signatures!
# gpg:          It is not certain that the signature belongs to the owner.
# Primary key fingerprint: 5379 2F71 024C 600F 778A  7161 D8D5 7199 DF83 42C8
#      Subkey fingerprint: F80C 0063 08E2 2CFD 8A92  E798 0CF5 572F D7FB 55AF

* remotes/aperard/tags/pull-xen-20190624:
  xen: Import other xen/io/*.h
  Revert xen/io/ring.h of "Clean up a few header guard symbols"
  xen: Drop includes of xen/hvm/params.h
  xen: Avoid VLA
  xen-bus / xen-block: add support for event channel polling
  xen-bus: allow AioContext to be specified for each event channel
  xen-bus: use a separate fd for each event channel
  xen-block: support feature-large-sector-size

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