conf: return backing information separately from metadata
    
    A couple pieces of virStorageFileMetadata are used only while
    collecting information about the chain, and don't need to
    live permanently in the struct.  This patch refactors external
    callers to collect the information separately, so that the
    next patch can remove the fields.
    
    * src/util/virstoragefile.h (virStorageFileGetMetadataFromBuf):
    Alter signature.
    * src/util/virstoragefile.c (virStorageFileGetMetadataInternal):
    Likewise.
    (virStorageFileGetMetadataFromFDInternal): Adjust callers.
    * src/storage/storage_backend_fs.c (virStorageBackendProbeTarget):
    Likewise.
    * src/storage/storage_backend_gluster.c
    (virStorageBackendGlusterRefreshVol): Likewise.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>