virStorageSourceGetMetadata: Use depth limit instead of unique path checking

virStorageSourceGetMetadata: Use depth limit instead of unique path checking

Prevent unbounded chains by limiting the recursion depth of
virStorageSourceGetMetadataRecurse to the maximum number of image layers
we limit anyways.

This removes the last use of virStorageSourceGetUniqueIdentifier which
will allow us to delete some crusty old infrastructure which isn't
really needed.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>