blkdebug: Don't require sophisticated filename

If the filename is not prefixed by "blkdebug:" in
blkdebug_parse_filename(), the blkdebug driver was not selected through
that protocol prefix, but by an explicit command line option
(file.driver=blkdebug or something similar). Contrary to the current
reaction, this is not a problem at all; we just need to store the
filename (in the x-image option) and can go on; the user just has to
manually specify the config option.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/blkdebug.c

@@ -313,7 +313,9 @@ static void blkdebug_parse_filename(const char *filename, QDict *options,
 
     /* Parse the blkdebug: prefix */
     if (!strstart(filename, "blkdebug:", &filename)) {
-        error_setg(errp, "File name string must start with 'blkdebug:'");
+        /* There was no prefix; therefore, all options have to be already
+           present in the QDict (except for the filename) */
+        qdict_put(options, "x-image", qstring_from_str(filename));
         return;
     }