qemu-ga update resetting VssOption Registry key to default
Description of problem:
Before I installed the .exe from iso `virtio-win-0.1.271.iso`, I had value 5 in registry key `HKLM:\SYSTEM\CurrentControlSet\Services\QEMU Guest Agent VSS Provider\VssOption`.
After the driver update by the .exe, the value was set to 1.

This registry key shouldn't change in driver update, as its value was manually set to 5 and it is important to preserve MSSQL backups in Proxmox.
Source:
https://blog.datact.ch/backup-mssql-server-with-proxmox
https://forum.proxmox.com/threads/pbs-breaking-customer-sql-backups-backups-without-fs-freeze.111526/
Steps to reproduce:
1. Set a value to `HKLM:\SYSTEM\CurrentControlSet\Services\QEMU Guest Agent VSS Provider\VssOption` other than 1.
2. Install the .exe from version 0.1.271.
3. Check the key value.