Choose which IP pool to use


I have just installed a new OpenNebula system based on Debian Stretch.
I have an /29 IPv4 subnet and an /64 IPv6 subnet.

Each of my VMs gets an IPv4 and an IPv6 address out of my subnets.
Since I only have a few IPv4 addresses, I want to deploy VMs with only(!) an IPv6 address.
So when creating a new VM I want to choose whether I want to use IPv4+IPv6 or just a single IPv6 address.

At first I thought I just have to add a new ip pool to the bridge, but that would mean, that I can only deploy IPv6 only VMs when my IPv4 pool is exhausted.
Anybody have an idea?