Merge remote-tracking branch 'remotes/sstabellini/tags/xsa155' into staging

XSA-155 fixes

# gpg: Signature made Fri 18 Dec 2015 15:16:18 GMT using RSA key ID 70E1AE90
# gpg: Good signature from "Stefano Stabellini <stefano.stabellini@eu.citrix.com>"

* remotes/sstabellini/tags/xsa155:
  xenfb: avoid reading twice the same fields from the shared page
  xen/blkif: Avoid double access to src->nr_segments

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