webadmin: Put back vNIC listener after flush

Since pressing the "Advanced Mode" button in the Add VM dialog causes
widgets to flush (without closing the dialog), the vNIC widget
listener should be put back to still be able to handle events after
the button was pressed.

Change-Id: I7cc1583bd629e0cf217d7e941f6fd018032831fb
Bug-Url: https://bugzilla.redhat.com/1029339
Signed-off-by: Lior Vernia <lvernia@redhat.com>