createnet: validate last page before creating the network

On the last page in the createnet wizard the Next button is changed to
Finish.  Ensure the user input is validated also on this page.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

virtManager/createnet.py

@@ -754,6 +754,9 @@ class vmmCreateNetwork(vmmGObjectUI):
         net.install()
 
     def finish(self, ignore):
+        if not self.validate(PAGE_MAX):
+            return
+
         try:
             net = self._build_xmlobj()
         except Exception, e: