lxc: eliminate extraneous free of netDef->ifname_guest
    
    lxcContainerRenameAndEnableInterfaces() isn't making a copy of the
    interface's ifname_guest (into newname), it's just copying the pointer
    to it. This means that when it later calls VIR_FREE(newname), it's
    actually freeing up (and fortunately NULLing out, so at least we don't
    try to access free'd memory) netDef->ifname_guest.