Ok i found i had this issue with my vm as well, all sorts of cpus would ramp up with doing a disk speedtest. Im using a r7 2700x on the gigabyte gaming 5 board and everything else works perfect. I picked up one of those samsung nvme drives on sale at best buy to test and with the drive passed through to the vm and cloned the vm to it....i no longer have the issue. I passed the entire controller through as well, no block device passthough. I see no cpus going crazy or anything.
beware 4k screenshot