This depends a bit on how your ISP gives your additional public IPs and how it wants them to be used inside on your network.
There are at least two possibilities:
1) Your ISP wants your primary public IP to act as a router, that routes packets to the final destination. That is, in the network layer 2, Ethernet frames containing IP packets for either of your public IPs are both delivered to the MAC address of your physical Ethernet interface.
Then it is the task of the dom0 to route the IP packets to your VM.
2) Your ISP offers you a normal Ethernet network (ISP router uses ARP to find out the MAC address of the IP address used by the VM). In this case you can just create a bridge that has your existing outside world interface as one member, and then the VMs use that bridge for their networking.
Then you need just to assign public IPs to the VMs and their ethernet devices that are connected to this bridge.