Use the normal virtio profile, everything just works
To get IPv6 to work, configure hostname.vio0 as follows:
dhcp
up
# next line is your IPv6 address as shown in vm show
inet6 2001:41c8:51:123:fcff:ff:feff:ffff 64
# This is the same for all bytemark hosts
!route add -inet6 default -ifp vio0 fe80::1%vio0
Use the Bytemark mirror for faster package installs, add the following to /etc/pkg.conf:
installpath=http://mirror.bytemark.co.uk/OpenBSD/5.6/packages/amd64