Previously, no one was using virStorageFileGetMetadata, and for good
reason - it couldn't support root-squash NFS.  Change the signature
and make it useful to future patches, including enhancing the metadata
to recursively track the entire chain.

* src/util/storage_file.h (_virStorageFileMetadata): Add field.
(virStorageFileGetMetadata): Alter signature.
* src/util/storage_file.c (virStorageFileGetMetadata): Rewrite.
(virStorageFileGetMetadataRecurse): New function.
(virStorageFileFreeMetadata): Handle recursion.