Now it's 2019 and this is still a problem :-)
I have been struggling with this for a week; I couldn't work out why ICMP from the host and another VM through the pfSense VM would work, but nothing else. I could only SSH into the host if I SSH to the pfSense VM first. In order to have the host be able to connect out I installed Squid and set it up as a transparent proxy, but I shouldn't have had to do this.
Researching, I finally found this thread. I'm replying because I just wanted to say that after I enabled "Disable hardware checksum offload" and pressed save, immediately traffic started flowing to/from the host, and the other VM which had basically been unreachable. No reboot or reconfig or anything else was required.
I now see it's fairly well documented here.. https://docs.netgate.com/pfsense/en/latest/virtualization/virtio-driver-support.html
Perhaps it would be nice if pfSense could automatically disable hardware checksum offload on the virtio driver/NICs :-)