interface: clean up virInterfaceDefDevFormat

This modifies the formatting function of virInterface to be a proper
mirror of the parse function, including the addition of a
"parentIfType" arg so that we can decide whether or not it is
appropriate to emit the elements that are only in toplevel interfaces,
as well as the <link> element (which isn't allowed for bridge
interfaces).

Since the restructuring of the code necessarily changes the order of
some of the elements, some test case data had to be updated.

tests/interfaceschemadata/vlan.xml

@@ -1,9 +1,9 @@
 <interface type='vlan' name='eth0.42'>
   <start mode='onboot'/>
-  <link state='lowerlayerdown'/>
   <protocol family='ipv4'>
     <dhcp peerdns='no'/>
   </protocol>
+  <link state='lowerlayerdown'/>
   <vlan tag='42'>
     <interface name='eth0'/>
   </vlan>