Merge remote-tracking branch 'remotes/aperard/tags/pull-xen-20190827' into staging

Xen queue

* Fixes for xen-bus and exit cleanup.
* Build fix.

# gpg: Signature made Tue 27 Aug 2019 14:31:10 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-20190827:
  xen-bus: Avoid rewriting identical values to xenstore
  xen-bus: Fix backend state transition on device reset
  xen: cleanup IOREQ server on exit
  xen: Fix ring.h header

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