storage: Switch metadata crawler to use storage driver to read headers
    
    Use virStorageFileReadHeader() to read headers of storage files possibly
    on remote storage to retrieve the image metadata.
    
    The backend information is now parsed by
    virStorageFileGetMetadataInternal which is now exported from the util
    source and virStorageFileGetMetadataFromFDInternal now doesn't need to
    be exported.