conf: provide details on network backing store
    
    So far, my work has been merely preserving the status quo of
    backing file analysis.  But this patch starts to tread in the
    territory of making the backing chain code more powerful - we
    will eventually support network storage containing non-raw
    formats.  Here, we expose metadata information about a network
    backing store, even if that information is still hardcoded to
    a raw format for now.
    
    * src/util/virstoragefile.c (virStorageFileGetMetadataRecurse):
    Also populate struct for non-file backing.
    (virStorageFileGetMetadata, virStorageFileGetMetadatainternal):
    Recognize non-file top image.
    (virFindBackingFile): Add comment.
    (virStorageFileChainGetBroken): Adjust comment, ensure output
    is set.
    * tests/virstoragetest.c (mymain): Update test to reflect it.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>