schema: add missing vlan element to networkport RNG

This is in the data structure and the parse/format functions, and is
getting passed all around correctly, it just was omitted from the RNG,
which hasn't been noticed because no human is creating <networkport>
XML, and so it's never getting validated against the schema.
Signed-off-by: NLaine Stump <laine@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

docs/schemas/networkport.rng

@@ -29,6 +29,9 @@
         <optional>
           <ref name="bandwidth"/>
         </optional>
+        <optional>
+          <ref name="vlan"/>
+        </optional>
         <optional>
           <ref name="plug"/>
         </optional>