Ubuntu 16.04 comes with xen-hypervisor 4.6, the next LTS-version Ubuntu 18.04 ships xen 4.9. Updates for meltdown will only published for xen 4.8 and 4.10. To protect your system with PVH against meltdown you have to build your xen-hypervisor from source.
pfsense 2.4.x is not available as nano-image, so you have to install pfsense as xen-Guest via vnc.
Sometimes you get the error
Out of resources when opening file './database/table.MYD' (Errcode: 24 - Too many open files)
To fix this on Ubuntu 16.04 you have to change the limits and to change the systemd-config: