XenAPI: Fix caching of images

Clearing of the cached images' name_label means it can't be found in future.
Name label of cloned VDI should be cleared so it is not found by the
caching functions.

Main impact of this is in test scenarios where the VM is always deleted, but
a more subtle impact could be data leakage.

Change-Id: I6bb80443d31128b7d5d47e4b252db5eb8ab86940
Closes-bug: 1254714
Merge "XenAPI: Fix caching of images"