conf: support configuring mtu size in a virtual network

Example:

  <network>
     ...
     <mtu size='9000'/>
     ...

If mtu is unset, it's assumed that we want the default for whatever is
the underlying transport (usually this is 1500).

This setting isn't yet wired in, so it will have no effect.

This partially resolves: https://bugzilla.redhat.com/1224348

docs/schemas/network.rng

@@ -79,6 +79,11 @@
           </element>
         </optional>
 
+        <!-- <mtu> element -->
+        <optional>
+          <ref name="mtu"/>
+        </optional>
+
         <!-- <mac> element -->
         <optional>
           <element name="mac">