network: always set disable_ipv6, even when it should be 0
    
    libvirt previously only touched an interface's disable_ipv6 setting in
    sysfs if it needed to be set to 1, assuming that 0 is the
    default. Apparently that isn't always the case though (kernel 3.15.7-1
    in Arch Linux reportedly defaults a new interface's disable_ipv6
    setting to 1) so this patch explicitly sets it to 0 or 1 as
    appropriate.