Specify prefix length for IPv6 subnets

Specify prefix length for IPv6 subnets

If Network contains dhcpv6-stateful Subnet with prefix other than /64,
dnsmasq refuses to start if prefix-len is not specified in dhcp-range
option.  From dnsmasq(8) manpage:

    For IPv6, the parameters are slightly different: instead of netmask
    and broadcast address, there is an optional prefix length which must
    be equal to or larger then the prefix length on the local interface.
    If not given, this defaults to 64. Unlike the IPv4 case, the prefix
    length is not automatically derived from the interface
    configuration. The mimimum size of the prefix length is 64.

It is safe to always specify prefix length, so dnsmasq will use correct
configuration.

Change-Id: Ibef1f55926585016f0054accd7f7ef5a022e3fe1
Closes-Bug: #1372883