storage/lvm: fix size reporting just after creating LV

Force cache refresh after registering new pool - it might be just
created.

QubesOS/qubes-issues#3438

qubes/storage/lvm.py

@@ -97,7 +97,8 @@ class ThinPool(qubes.storage.Pool):
         return volume
 
     def setup(self):
-        pass  # TODO Should we create a non existing pool?
+        reset_cache()
+        # TODO Should we create a non existing pool?
 
     def get_volume(self, vid):
         ''' Return a volume with given vid'''