schema: Allow shmem to have alias

It already is used and tests will be automatically added in later patches.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

docs/schemas/domaincommon.rng

@@ -3611,6 +3611,9 @@
             </optional>
           </element>
         </optional>
+        <optional>
+          <ref name="alias"/>
+        </optional>
         <optional>
           <ref name="address"/>
         </optional>