This happened to me yesterday when I launched a windows VM that I have been using for about 3 months now. I am running 2 windows vm's both seemed to fill up the log and shut down.
After searching online and doing some trouble shooting-with no luck- I decided to delete all vm's,vdisk, libvirt.image and starting over.
The problem did not happen when setting up my first windows 10 vm which I only use though vnc.
The second windows vm with gpu passthrough is where I get the problem so I'm guessing it has to do with that. So I tried to reinstall NVidia driver plugin updated and made sure it sees my GPU 1070ti.
After a reboot all returns to normal. the issue only comes back when that VM is turned on.
Im running the newest unraid os build everything was running fine after update.
I'm attaching the diagnostics file here.
tower-diagnostics-20210324-1943.zip