network: fix DHCPv6 on networks with prefix != 64

network: fix DHCPv6 on networks with prefix != 64

According to the dnsmasq manpage, the netmask for IPv4 address ranges
will be auto-deteremined from the interface dnsmasq is listening on,
but it can't do this for IPv6 for some reason - it instead assumes a
network prefix of 64 for all IPv6 address ranges. If this is
incorrect, dnsmasq will refuse to give out an address to clients,
instead logging this message:

 dnsmasq-dhcp[2380]: no address range available for DHCPv6 request via virbr0

The solution is for libvirt to add ",$prefix" to all IPv6 dhcp-range
arguments when building the dnsmasq.conf file.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1033739