Blacklisting ahci would get you into trouble (since all SATA cotnrolers need it) but softdep should do the trick. Something likesoftdep ahci pre: vfio-pci
(added to /etc/modprobe.d/vfio.conf and runningupdate-initramfs -u
) will simply load vfio-pci (just) before ahci loads. Check withlspci -nnk
after a fresh reboot without starting the VM to make sure the driver in use is vfio-pci.
PS: Feel free to make a separate thread about this if you run into issues and need help.
This worked out like a charm. Thank you very much!

Unbelievable how often I did read the
softdep
-option in your posts and never came to the idea, that I should use it in my own setup; stupid me. 
Hopefully the stack trace came either from this, maybe in combination with changes in that kernel-version or that it at least was only a one-time thing.