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

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 <lvivier@redhat.com>
Cc: Amit Shah <amit@kernel.org>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
(mjt: drop superfluous commit as suggested by philmd)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>