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>