https://forum.proxmox.com/threads/unknown-device-in-win7-vm.49698/#post-246732

https://forum.proxmox.com/threads/device-pci-simple-communications.41502/

https://www.microsoft.com/en-us/download/details.aspx?id=48204

https://support.microsoft.com/en-us/topic/hyper-v-integration-components-update-for-windows-virtual-machines-that-are-running-on-a-windows-10-or-windows-server-2016-based-host-bd22f4f6-feec-89f4-8a1d-405076ff4222

TLDR for installing the update:

1.) Download the Integration Services from Microsoft Download Center:

2.) Open a Powershell with Admin-rights

3.) Execute this:

Dism /online /Add-Package /PackagePath:C:packagespackage.cab

(You have to change the path to the path where you put the downloaded cab file)