Disables non-O_DIRECT workaround

[Xen-changelog] [xen master] libxl: introduce an option for disabling the non-O_DIRECT workaround
http://lists.xen.org/archives/html/xen-changelog/2014-05/msg00136.html

Description:           Disables non-O_DIRECT workaround
There is a memory lifetime bug in some driver domain (dom0) kernels
which can cause crashes when using O_DIRECT.  The bug occurs due to a
mismatch between the backend-visible lifetime of pages used for the
Xen PV network protocol and that expected by the backend kernel's
networking subsystem.  This can cause crashes when using certain
backends with certain underlying storage.
See:
http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html