doc: Add schema definition for imagelabel

<imagelable> is not generated by running domain, actually we parse
it in src/conf/domain_conf.c, this patch is to fix it, otherwise any
validation (virt-xml-validate) on the domain xml dumped from shutoff
domain containing <imagelable> will fail.

* docs/schemas/domain.rng

docs/schemas/domain.rng

@@ -64,6 +64,9 @@
       <element name="label">
         <text/>
       </element>
+      <element name="imagelabel">
+        <text/>
+      </element>
     </element>
   </define>
   <define name="hvs">