Hi Sandro,
Isn't guest agent installed with guest tools?
I have a similar situation with Windows 2016 or Windows 10 (64 bit) where although I have installed oVirt Guest Tools 4.1-5.fc24 (and seems working as apps are listed, IPs are shown, etc) when going to perform a snapshot it mentions:
Could not detect Guest Agent on the VM. Note that without a Guest Agent the data on the created snapshot may be inconsistent.
The above issue is also with 4.2 version of guest tools.
What do we need to check further?
Thanx,
Alex