Install FreeBSD from source into VM image

Hello everybody!

I am working on the FreeBSD test suite, and need to create an image file 
from source.  How can I do that?

I need to run something similar to make installkernel && make 
installworld with an image file as the target, such that the end result 
is a ready-made FreeBSD system that can be started up with bhyve.  How 
can I do that, including creating the correct /etc files, and the 
correct boot code and partitioning?

Following the FreeBSD handbook [0] didn't work, as that concerns 
updating the current system from source only.

     [0]: https://www.freebsd.org/doc/handbook/makeworld.html

Any help would be kindly appreciated!

Best regards,
Panagiotes