Merge remote-tracking branch 'remotes/aperard/tags/pull-xen-20190204' into staging

Xen queue

* xen-block, the Xen PV backend, now handles resize.
* configure cleanup.
* xen-bus fix.

# gpg: Signature made Mon 04 Feb 2019 11:16:13 GMT
# 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-20190204:
  xen-block: handle resize callback
  xen: fix xen-bus state model to allow frontend re-connection
  configure: Don't add Xen's libs to LDFLAGS
  configure: xen: Stop build-testing for xc_domain_create

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