blkdebug: Use error_setg_file_open() for a better error message

The error message changes from

    Could not read blkdebug config file: REASON

to

    Could not open 'FNAME': REASON

I think the exact file name is more useful to know than the file's
purpose.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20251121121438.1249498-10-armbru@redhat.com>

block/blkdebug.c

@@ -288,7 +288,7 @@ static int read_config(BDRVBlkdebugState *s, const char *filename,
     if (filename) {
         f = fopen(filename, "r");
         if (f == NULL) {
-            error_setg_errno(errp, errno, "Could not read blkdebug config file");
+            error_setg_file_open(errp, errno, filename);
             return -errno;
         }