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 <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

taget/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,taget/libvirt,nertpinx/libvirt,shugaoye/libvirt,libvirt/libvirt,andreabolognani/libvirt,jfehlig/libvirt,jfehlig/libvirt,libvirt/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,elmarco/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,olafhering/libvirt,nertpinx/libvirt,eskultety/libvirt,shugaoye/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,rlaager/libvirt,olafhering/libvirt,fabianfreyer/libvirt,zippy2/libvirt,crobinso/libvirt,crobinso/libvirt,datto/libvirt,nertpinx/libvirt,agx/libvirt,crobinso/libvirt,jfehlig/libvirt,eskultety/libvirt,nertpinx/libvirt,andreabolognani/libvirt,datto/libvirt,fabianfreyer/libvirt,taget/libvirt,zippy2/libvirt,crobinso/libvirt,elmarco/libvirt,rlaager/libvirt,jardasgit/libvirt,agx/libvirt,datto/libvirt,elmarco/libvirt,jardasgit/libvirt,eskultety/libvirt,shugaoye/libvirt,elmarco/libvirt,datto/libvirt,rlaager/libvirt,andreabolognani/libvirt,nertpinx/libvirt,agx/libvirt,elmarco/libvirt,agx/libvirt,taget/libvirt,olafhering/libvirt,eskultety/libvirt,libvirt/libvirt,andreabolognani/libvirt,shugaoye/libvirt,eskultety/libvirt,VenkatDatta/libvirt,libvirt/libvirt,taget/libvirt,rlaager/libvirt,agx/libvirt,datto/libvirt,olafhering/libvirt,zippy2/libvirt,rlaager/libvirt,zippy2/libvirt,shugaoye/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,jardasgit/libvirt,jardasgit/libvirt