Hi Valerio!,
if you don’t mind using VirtIO drivers which are recommended for enhanced performance when using KVM, I’d have a look at this post which explains how to install Windows 7 in OpenNebula. If you need help just post here what you need.
Your win7-empty-hd is configured with the DEV_PREFIX=“hd” which should be an IDE emulated disk and Windows should see it but it’s been a long time since I installed Windows in OpenNebula.
Cheers!