Merge remote-tracking branch 'sstabellini/xen-2013-04-05' into staging

# By Alex Bligh (2) and Felipe Franciosi (2)
# Via Stefano Stabellini
* sstabellini/xen-2013-04-05:
  Allow xen guests to plug disks of 1 TiB or more
  Introduce 64 bit integer write interface to xenstore
  Xen PV backend: Disable use of O_DIRECT by default as it results in crashes.
  Xen PV backend: Move call to bdrv_new from blk_init to blk_connect