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

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: Eric Blake <eblake@redhat.com>