XendAPIStore - do not remove non-existent item

If list is searched and item not found, then don't attempt to delete
the item.

Signed-off-by: Jim Fehlig <jfehlig@novell.com>

tools/python/xen/xend/XendAPIStore.py

@@ -33,7 +33,8 @@ def register(uuid, type, inst):
 
 def deregister(uuid, type):
     old = get(uuid, type)
-    del __classes[(uuid, type)]
+    if old is not None:
+        del __classes[(uuid, type)]
     return old
 
 def get(uuid, type):