network: always set disable_ipv6, even when it should be 0

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.