util: don't log failure when older iptables lacks -w

Commit dc33e6e4 caused older platforms like Fedora 20 to emit
scary log messages at startup:

2014-11-19 23:12:58.800+0000: 28906: error : virCommandWait:2532 : internal error: Child process (/usr/sbin/iptables -w -L -n) unexpected exit status 2: iptables v1.4.19.1: unknown option "-w"
Try `iptables -h' or 'iptables --help' for more information.

Since we are probing and expect to handle the case where -w is not
supported, we should not let virCommand log it as an error.

* src/util/virfirewall.c (virFirewallCheckUpdateLock): Handle
non-zero status ourselves.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/util/virfirewall.c

@@ -112,8 +112,9 @@ static void
 virFirewallCheckUpdateLock(bool *lockflag,
                            const char *const*args)
 {
+    int status; /* Ignore failed commands without logging them */
     virCommandPtr cmd = virCommandNewArgs(args);
-    if (virCommandRun(cmd, NULL) < 0) {
+    if (virCommandRun(cmd, &status) < 0 || status) {
         VIR_INFO("locking not supported by %s", args[0]);
     } else {
         VIR_INFO("using locking for %s", args[0]);