schema: capabilities: Add 'hap' feature flag

The libxl driver exposes a 'hap' feature in the capability XML but our
schema didn't cover it.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

docs/schemas/capability.rng

@@ -546,6 +546,12 @@
             <empty/>
           </element>
         </optional>
+        <optional>
+          <element name='hap'>
+            <ref name='featuretoggle'/>
+            <empty/>
+          </element>
+        </optional>
       </interleave>
     </element>
   </define>