# Install if lshw not found
apt-get update
apt install lshw

# Check disk info
lshw -class disk -class storage

# Do this again for every disk
qm set <disk_id> -virtio<number from 2 to 999, remember that you cannot use 1!> /dev/disk/by-id/<disk_long_id>
# (this prints: update VM <disk_id> -virtio2 /dev/disk/by-id/<disk_long_id>)

# Check if it did work
cat /etc/pve/qemu-server/<disk_id>.conf

# (output is something like this:
# virtio2: /dev/disk/by-id/<disk_long_id>,size=1953514584K
# virtio3: /dev/disk/by-id/<disk_long_id>,size=2930266584K
# virtio4: /dev/disk/by-id/<disk_long_id>,size=2930266584K
# )

# As always when you change the hardware for a KVM guest, do a poweroff and a start (reboot is not enough).