conf: delete internal directory field

conf: delete internal directory field

Another field no longer needed, getting us one step closer to
merging virStorageFileMetadata and virStorageSource.

* src/util/virstoragefile.h (_virStorageFileMetadata): Drop
field.
* src/util/virstoragefile.c (virStorageFileGetMetadataInternal)
(virStorageFileGetMetadataFromFDInternal): Alter signature.
(virStorageFileFreeMetadata, virStorageFileGetMetadataFromBuf)
(virStorageFileGetMetadataFromFD): Adjust clients.
* tests/virstoragetest.c (_testFileData, testStorageChain)
(mymain): Simplify test.

Signed-off-by: Eric Blake <eblake@redhat.com>