docs: make interface start mode element optional

This brings the libvirt version of this RNG file in line with the same
file in netcf (as soon as the corresponding patch there is ACKed and
pushed).

There's no reason to require it when defining an interface (the config
option it corresponds to is optional), and it isn't even output in the
status of an interface.

Resolves: https://bugzilla.redhat.com/1414404

docs/schemas/interface.rng

@@ -279,6 +279,7 @@
   </define>
 
   <define name="startmode">
+    <optional>
       <element name="start">
         <attribute name="mode">
           <choice>
@@ -290,6 +291,7 @@
           </choice>
         </attribute>
       </element>
+    </optional>
   </define>
 
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