Hello,
I am a new proxmox free user and wanted to say that this :
qm set ID -args '-machine type=q35,kernel_irqchip=on'
It has made my VMs with GeForce 970 (UEFI) and GeForce 560 Ti (Seabios) PCIe Passthrough GPUs work. Finally.
Without this bit it would fail right after driver install...