To set or alter you Reverse DNS you will need to login to the SolusVM control panel located at http://vps.xenserver.co.uk (login details in your VPS activation email)
You will then be able to alter the rDNS against the IP under the 'Network' tab. The hostname set for the rDNS entry MUST be a FQDN (Fully Qualified Domain Name).
These can be set for both IPv4 and IPv6