If the ipv6 kernel module is not loaded, then we get this when starting
a virtual network:

  libvir: Network Config error :
  cannot enable /proc/sys/net/ipv6/conf/virbr0/disable_ipv6:
  No such file or directory

If disable_ipv6 is not present, we should just merrily continue on our
way.

* src/network_driver.c: make networkDisableIPV6() not fail if the kernel
  has no ipv6 support