devices/lvm2.lv: initialize fullname member

This gets set in `open` and used in `close` but if the former
fails the latter will explode if we do not properly initialize
it. Also, we should always properly initialize things.

devices/org.osbuild.lvm2.lv

@@ -56,6 +56,7 @@ class LVService(devices.DeviceService):
 
     def __init__(self, args):
         super().__init__(args)
+        self.fullname = None
         self.target = None
 
     @staticmethod