qemu: Support discard for disk

QEMU introduced "discard" option for drive since commit a9384aff53,

<...>
@var{discard} is one of "ignore" (or "off") or "unmap" (or "on") and
controls whether @dfn{discard} (also known as @dfn{trim} or @dfn{unmap})
requests are ignored or passed to the filesystem.  Some machine types
may not support discard requests.
</...>

This patch exposes the support in libvirt.

QEMU supported "discard" for "-drive" since v1.5.0-rc0:

% git tag --contains a9384aff53
contains
v1.5.0-rc0
v1.5.0-rc1

So this only detects the capability bit using virQEMUCapsProbeQMPCommandLine.

docs/formatdomain.html.in

@@ -1702,6 +1702,14 @@
             network. By default copy-on-read is off.
             <span class='since'>Since 0.9.10 (QEMU and KVM only)</span>
           </li>
+          <li>
+            The optional <code>discard</code> attribute controls whether
+            to discard (also known as "trim" or "unmap") requests are
+            ignored or passed to the filesystem. The value can be either
+            "on" (allow the discard request to be passed) or "off" (ingore
+            the discard request).
+            <span class='since'>Since 1.0.6 (QEMU and KVM only)</span>
+          </li>
         </ul>
       </dd>
       <dt><code>boot</code></dt>

docs/schemas/domaincommon.rng

@@ -1311,6 +1311,9 @@
       <optional>
         <ref name="copy_on_read"/>
       </optional>
+      <optional>
+        <ref name="discard"/>
+      </optional>
       <empty/>
     </element>
   </define>
@@ -1406,6 +1409,14 @@
         <value>off</value>
       </choice>
     </attribute>
+    </define>
+  <define name="discard">
+    <attribute name='discard'>
+      <choice>
+        <value>on</value>
+        <value>off</value>
+      </choice>
+    </attribute>
   </define>
   <define name="controller">
     <element name="controller">