Started having the same issue with my 3070 passthrough to Win10 VM after I reflashed my unraid thumbdrive due to corruption.
Tried all the guides I could find online, tired disabling CSM, setting UEFI only, setting bind iommu group to vfio at boot, edited xml.... nothing worked.
Finally decided to shutdown the system, pull the plug, press the power to drain board, unplugged hdmi from 3070, re-attached power, and started system.
I waited until unraid had fully booted, then I started the VM (was set to auto start), then attached the hdmi back to the rtx... BLAM, win10 VM was on screen.
I'll update this once I've got a more reliable method, in the meantime I hope this helps someone avoid a headache! ^.^