Add directsync cache mode support for disk driver

Newer QEMU introduced cache=directsync for -drive, this patchset
is to expose it in libvirt layer.

  * Introduced a new QEMU capability flag ($prefix_CACHE_DIRECTSYNC),
    As even $prefix_CACHE_V2 is set, we can't known if directsync
    is supported.

docs/formatdomain.html.in

@@ -976,7 +976,9 @@
           <li>
             The optional <code>cache</code> attribute controls the
             cache mechanism, possible values are "default", "none",
-            "writethrough" and "writeback".
+            "writethrough", "writeback", and "directsync". "directsync"
+            is like "writethrough", but it bypasses the host page
+            cache.
             <span class="since">Since 0.6.0</span>
           </li>
           <li>

docs/schemas/domain.rng

@@ -829,6 +829,7 @@
         <value>none</value>
         <value>writeback</value>
         <value>writethrough</value>
+        <value>directsync</value>
       </choice>
     </attribute>
   </define>