With the issue being "it doesn't work".
The virtual machines come up just fine; I can ssh into them and do all sorts of things that way.
Installing a new one or attempting to use the graphical display is another story. The display freezes after a partial draw and there's no keyboard input allowed. I've used both SPICE and VNC to bring up the display and neither works.
I'm using the latest versions out of slackbuilds.org to build the infrastructure. I'm using sbopkg to build it all via a queue file that looks like...
...and I'm at my wit's end (which isn't a long journey). It all worked fine with the versions used in 14.0.