addons: vlan: validate vlan interface name

we have an automation test that adds a vrf named "VRF.TEST":

$ ifquery -a
auto VRF.TEST
iface VRF.TEST
    vrf-table auto

$ ifup -a
error: could not determine vlanid
warning: error while writing to file /sys/class/net/VRF.TEST/mtu: [Errno 22] Invalid argument
$

Signed-off-by: Julien Fortin <jfortin@nvidia.com>

ifupdown2/addons/vlan.py

@@ -64,13 +64,16 @@ class vlan(Addon, moduleBase):
         Addon.__init__(self)
         moduleBase.__init__(self, *args, **kargs)
 
-    @staticmethod
-    def _is_vlan_device(ifaceobj):
+    def _is_vlan_device(self, ifaceobj):
         vlan_raw_device = ifaceobj.get_attr_value_first('vlan-raw-device')
         if vlan_raw_device:
             return True
         elif '.' in ifaceobj.name:
-            return True
+            try:
+                if self._get_vlan_id(ifaceobj) != -1:
+                    return True
+            except:
+                pass
         return False
 
     @staticmethod