Merge remote-tracking branch 'remotes/famz/tags/build-and-test-pull-request' into...

# gpg: Signature made Tue 15 Aug 2017 11:50:36 BST
# gpg:                using RSA key 0xCA35624C6A9171C6
# gpg: Good signature from "Fam Zheng <famz@redhat.com>"
# 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: 5003 7CB7 9706 0F76 F021  AD56 CA35 624C 6A91 71C6

* remotes/famz/tags/build-and-test-pull-request:
  docker: add centos7 image
  docker: install more packages on CentOS to extend code coverage
  docker: add Xen libs to centos6 image
  docker: use one package per line in CentOS config
  Makefile: Let "make check-help" work without running ./configure

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