Replace fprintf() by qemu_log_mask(LOG_GUEST_ERROR).

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200901104234.92159-2-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>