Hi Dominic,
note that ASA with FirePOWER services includes a special backplane integration, that is not exposed to virtual ASA. This means that you would not be able to emulate it fully in virtual form. I.e., you would not be able to use MPF to direct traffic to NGIPS, it would either be all traffic or none.
What you could do, is use VLAN stitching with ASAv and NGIPSv, and do something that is described in this video:
Resilient NGFW Design Chalk Talk - YouTube
Hope this helps.
Thanks,
Goran