iptables: don't log command probe failures

Commit b9dd878f caused a regression in iptables interaction by
logging non-zero status at a higher level than VIR_INFO.  Revert
that portion of the commit, as well as adding a comment explaining
why we check the status ourselves.

Reported by Nehal J Wani.

* src/util/viriptables.c (virIpTablesOnceInit): Undo log regression.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/util/viriptables.c

@@ -60,6 +60,7 @@ static int
 virIpTablesOnceInit(void)
 {
     virCommandPtr cmd;
+    int status;
 
 #if HAVE_FIREWALLD
     firewall_cmd_path = virFindFileInPath("firewall-cmd");
@@ -70,7 +71,8 @@ virIpTablesOnceInit(void)
         cmd = virCommandNew(firewall_cmd_path);
 
         virCommandAddArgList(cmd, "--state", NULL);
-        if (virCommandRun(cmd, NULL) < 0) {
+        /* don't log non-zero status */
+        if (virCommandRun(cmd, &status) < 0 || status != 0) {
             VIR_INFO("firewall-cmd found but disabled for iptables");
             VIR_FREE(firewall_cmd_path);
             firewall_cmd_path = NULL;
@@ -87,7 +89,8 @@ virIpTablesOnceInit(void)
 
     cmd = virCommandNew(IPTABLES_PATH);
     virCommandAddArgList(cmd, "-w", "-L", "-n", NULL);
-    if (virCommandRun(cmd, NULL) < 0) {
+    /* don't log non-zero status */
+    if (virCommandRun(cmd, &status) < 0 || status != 0) {
         VIR_INFO("xtables locking not supported by your iptables");
     } else {
         VIR_INFO("using xtables locking for iptables");