XenAPI: Fix bug with Xen ext4-using guests

Ubuntu saucy is using ext4, which means it hits a barrier
bug with certain versions of Xen, leading to a read only
filesystem.

This is bug https://bugs.launchpad.net/ubuntu/+source/linux/+bug/824089

Change-Id: I9a72b203d473dc555324d44ad7c240c80dccda15