FreeNAS 8 runs nicely under Linux KVM virtualisation. But unlike FreeNAS 7, FreeNAS 8 is based on FreeBSD 8.2. This means that there are some configuration differences. The main one is the network device model type='ne2k_pci' does not seem to work for FreeBSD 8.2 or FreeNAS 8.
So if you're having trouble getting the networking to work, try using model type='rtl8139'. And you will need at least 1GB for the OS drive, otherwise the installation will fail silently.
Below is a working scripted install that we used on one of our lab computers to successfully ...