A later patch will be adding some new fields to
virStorageFileMetadata; to minimize confusion, renaming the
test fields now will make it more obvious which fields are
being tested later.

* tests/virstoragetest.c (_testFileData): Alter names.
(testStorageChain, mymain): Adjust clients.
Signed-off-by: NEric Blake <eblake@redhat.com>