conf: track user vs. canonical name through full chain lookup

conf: track user vs. canonical name through full chain lookup

The previous patch started a separation of error messages
reported against the user-specified name, vs. tracking the
canonical path that was actually opened.  This patch extends
that notion, by hoisting directory detection up front, passing
the canonical path through the entire call chain, and
simplifying lower-level functions that can now assume that
a canonical path and directory have been supplied.

* src/util/virstoragefile.c
(virStorageFileGetMetadataFromFDInternal)
(virStorageFileGetMetadataInternal): Add parameter, require
directory.
(virFindBackingFile): Require directory.
(virStorageFileGetMetadataFromFD): Pass canonical path.
(virStorageFileGetMetadataFromBuf): Likewise.
(virStorageFileGetMetadata): Determine initial directory.

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