qemu: report whether a machine type is deprecated in capabilities

QEMU has the ability to mark machine types as deprecated. This should be
exposed to management applications in the capabilities.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

docs/schemas/capability.rng

@@ -407,6 +407,14 @@
           <ref name="unsignedInt"/>
         </attribute>
       </optional>
+      <optional>
+        <attribute name="deprecated">
+          <choice>
+            <value>yes</value>
+            <value>no</value>
+          </choice>
+        </attribute>
+      </optional>
       <text/>
     </element>
   </define>