Merge remote-tracking branch 'sstabellini/xen-130729' into staging

# By Paul Durrant (1) and Stefano Stabellini (1)
# Via Stefano Stabellini
* sstabellini/xen-130729:
  Xen PV Device
  xen_disk: support "direct-io-safe" backend option

Message-id: 1375096790-12815-1-git-send-email-stefano.stabellini@eu.citrix.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>