I got Qubes 4 RC-2 running on a similar motherboard for someone else, the Asus z170 Pro Gaming. I don't use this system actively my self, but it ran smoothly from the hours I used to set it all up and to configure it, and it had no libvirt errors.
So the basic chipset features of the z170 line should not be an issue, and you got immou enabled in your HCL rapport file, so I assume your VT-d must be enabled in your UEFI (BIOS). Also the processor doesn't look like an issue either.
However, I remember this particular motherboard had two separate places to activate the virtualisation technology, you may have only enabled one of them in the UEFI? Both settings came disabled on the Asus z170 Pro Gaming board I set up with Qubes 4.
In your UEFI Advanced menu, try look for both:
- VT-d. (I suspect this is the one you enabled).
- Virtualization (something closely worded to this, I suspect this might be the setting you miss to enable).
So if the two boards are anything alike, then this might be it. If you can't find this extra setting, then perhaps your model is different and it doesn't have it extra required step. But it may very well be similar.
Yes, there are two places to enable on this motherboard, both in Advanced settings, one under CPU configuration and another one under Chipset configuration and they are both enabled.