CA-136654: Don't rely on a XAPI session existing

With static VDIs, the SR code is run without a XAPI session.  In this
case, set O_DIRECT to be on for safety since there is external knob to
adjust it.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #142 on xapi-project/sm
(cherry picked from commit 6c2d966ee492d756aa90dae84a0fc2298d4d4002)