conf: add support for Hyper-V Enlightened VMCS

Support Hyper-V Enlightened VMCS in domain config. QEMU support will
be implemented in the next patch, adding interim VIR_DOMAIN_HYPERV_EVMCS
cases to src/qemu/* for now.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>

docs/formatdomain.html.in

@@ -2008,6 +2008,7 @@
     <reenlightenment state='on'/>
     <tlbflush state='on'/>
     <ipi state='on'/>
+    <evmcs state='on'/>
   </hyperv>
   <kvm>
     <hidden state='on'/>
@@ -2154,6 +2155,12 @@
           <td>on, off</td>
           <td><span class="since">4.10.0 (QEMU 3.1)</span></td>
         </tr>
+        <tr>
+          <td>evmcs</td>
+          <td>Enable Enlightened VMCS</td>
+          <td>on, off</td>
+          <td><span class="since">4.10.0 (QEMU 3.1)</span></td>
+        </tr>
       </table>
       </dd>
       <dt><code>pvspinlock</code></dt>