qemu: support for kvm-hint-dedicated performance hint

QEMU version 2.12.1 introduced a performance feature under commit
be7773268d98 ("target-i386: add KVM_HINTS_DEDICATED performance hint")

This patch adds a new KVM feature 'hint-dedicated' to set this performance
hint for KVM guests. The feature is off by default.

To enable this hint and have libvirt add "-cpu host,kvm-hint-dedicated=on"
to the QEMU command line, the following XML code needs to be added to the
guest's domain description in conjunction with CPU mode='host-passthrough'.

  <features>
    <kvm>
      <hint-dedicated state='on'/>
    </kvm>
  </features>
  ...
  <cpu mode='host-passthrough ... />

Signed-off-by: Wim ten Have <wim.ten.have@oracle.com>
Signed-off-by: Menno Lageman <menno.lageman@oracle.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

docs/formatdomain.html.in

@@ -2044,6 +2044,7 @@
   </hyperv>
   <kvm>
     <hidden state='on'/>
+    <hint-dedicated state='on'/>
   </kvm>
   <pvspinlock state='on'/>
   <gic version='2'/>
@@ -2217,6 +2218,12 @@
           <td>on, off</td>
           <td><span class="since">1.2.8 (QEMU 2.1.0)</span></td>
         </tr>
+        <tr>
+          <td>hint-dedicated</td>
+          <td>Allows a guest to enable optimizations when running on dedicated vCPUs</td>
+          <td>on, off</td>
+          <td><span class="since">5.7.0 (QEMU 2.12.1)</span></td>
+        </tr>
       </table>
       </dd>
       <dt><code>pmu</code></dt>