vmdk: Fix error for JSON descriptor file names

vmdk: Fix error for JSON descriptor file names

If vmdk blindly tries to use path_combine() using bs->file->filename as
the base file name, this will result in a bad error message for JSON
file names when calling bdrv_open(). It is better to only try
bs->file->exact_filename; if that is empty, bs->file->filename will be
useless for path_combine() and an error should be emitted (containing
bs->file->filename because desc_file_path (which is
bs->file->exact_filename) is empty).

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Message-id: 1417615043[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>