Introduce SMM feature

Since its release of 2.4.0 qemu is able to enable System
Management Module in the firmware, or disable it. We should
expose this capability in the XML. Unfortunately, there's no good
way to determine whether the binary we are talking to supports
it. I mean, if qemu's run with real machine type, the smm
attribute can be seen in 'qom-list /machine' output. But it's not
there when qemu's run with -M none. Therefore we're stuck with
version based check.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

docs/formatdomain.html.in

@@ -1680,6 +1680,12 @@
           values are <code>2</code>, <code>3</code> and <code>host</code>.
           <span class="since">Since 1.2.16</span>
       </dd>
+      <dt><code>smm</code></dt>
+      <dd>Enable System Management Mode. Possible values are
+          <code>on</code> and <code>off</code>. The default is left
+          for hypervisor to decide.
+          <span class="since">Since 2.1.0</span>
+      </dd>
     </dl>
 
     <h3><a name="elementsTime">Time keeping</a></h3>

docs/schemas/domaincommon.rng

@@ -4325,6 +4325,15 @@
               </optional>
             </element>
           </optional>
+          <optional>
+            <element name="smm">
+              <optional>
+                <attribute name="state">
+                  <ref name="virOnOff"/>
+                </attribute>
+              </optional>
+            </element>
+          </optional>
         </interleave>
       </element>
     </optional>