Merge remote-tracking branch 'remotes/sstabellini/tags/2015-10-19-tag' into staging

Xen 2015-10-19

# gpg: Signature made Mon 19 Oct 2015 11:24:05 BST using RSA key ID 70E1AE90
# gpg: Good signature from "Stefano Stabellini <[email protected]>"

* remotes/sstabellini/tags/2015-10-19-tag:
  xen-platform: Ensure xen is enabled when initializing
  pc: Require xen when initializing xenfv machine

Signed-off-by: Peter Maydell <[email protected]>