log: Open log file with O_APPEND

Otherwise multiple writers can screw each-other's messages.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

log.c

@@ -39,7 +39,7 @@ int log_init(const char *output)
 	}
 
 	if (output) {
-		new_logfd = open(output, O_CREAT | O_WRONLY, 0600);
+		new_logfd = open(output, O_CREAT | O_WRONLY | O_APPEND, 0600);
 		if (new_logfd < 0) {
 			pr_perror("Can't create log file %s", output);
 			return -1;