docs: schema: Add missing <alias> to vsock device

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1600345Signed-off-by: NHan Han <hhan@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

docs/schemas/domaincommon.rng

@@ -4234,6 +4234,9 @@
         <optional>
           <ref name="address"/>
         </optional>
+        <optional>
+          <ref name="alias"/>
+        </optional>
       </interleave>
     </element>
   </define>