Report VNC tag only when valid.

When VM is not started, vnc port value is -1 in the libvirt domain XML config.

ccnode/hypervisor/domains/vm_tags.py

@@ -92,11 +92,17 @@ def memmax(dom):
 def vncport(dom):
     """VNC port for the VM console access."""
     try:
-        return et.ElementTree().parse(
+        port = et.ElementTree().parse(
             StringIO(dom.lv_dom.XMLDesc(0))
         ).find('devices/graphics').get('port')
     except Exception:
         logger.exception('VNCPort')
+        raise
+
+    if 0 < port < 65536:
+        return port
+
+    return
 
 
 @ttl(10)