Rename iterface type='dummy' to type='null'

When commit bac6b266fb added this "functionality" this was the only
naming I could think of, but after discussion with Dan we found the name
'null' fits a bit better, so change it before we make a release with the
old name.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

tests/xml2vmxdata/xml2vmx-ethernet-null.xml

@@ -6,7 +6,7 @@
     <type>hvm</type>
   </os>
   <devices>
-    <interface type='dummy'>
+    <interface type='null'>
       <mac address='00:50:56:87:65:43'/>
     </interface>
   </devices>