In order to search for a backing file name as literally present
in a chain, we need to remember if the chain had relative names.
Also, searching for absolute names is easier if we only have
to canonicalize once, rather than on every iteration.

* src/util/storage_file.h (_virStorageFileMetadata): Add field.
* src/util/storage_file.c (virStorageFileGetMetadataFromBuf):
(virStorageFileFreeMetadata): Manage it
(absolutePathFromBaseFile): Store absolute names in canonical form.