backends/rng-random: Get rid of qemu_open_old()

For qemu_open_old(), osdep.h said:

> Don't introduce new usage of this function, prefer the following
> qemu_open/qemu_create that take an "Error **errp".

So replace qemu_open_old() with qemu_open(). And considering
rng_random_opened() will lose its obvious error handling case after
removing error_setg_file_open(), add comment to remind here.

Cc: Laurent Vivier <[email protected]>
Cc: Amit Shah <[email protected]>
Signed-off-by: Zhao Liu <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Michael Tokarev <[email protected]>
(mjt: drop superfluous commit as suggested by philmd)
Signed-off-by: Michael Tokarev <[email protected]>

backends/rng-random.c

@@ -75,10 +75,7 @@ static void rng_random_opened(RngBackend *b, Error **errp)
         error_setg(errp, QERR_INVALID_PARAMETER_VALUE,
                    "filename", "a valid filename");
     } else {
-        s->fd = qemu_open_old(s->filename, O_RDONLY | O_NONBLOCK);
-        if (s->fd == -1) {
-            error_setg_file_open(errp, errno, s->filename);
-        }
+        s->fd = qemu_open(s->filename, O_RDONLY | O_NONBLOCK, errp);
     }
 }