Hide hypervisor signature on Windows guests for nvidia GPU

Blueprints hide-hypervisor-id-flavor-extra-spec and add-kvm-hidden-feature allow hiding KVM's signature for guests, which is necessary for Nvidia drivers to work in VMs with passthrough GPUs.
While this works well for linux guests on KVM hypervisors, it doesn't work for Windows guests on KVM. For them, KVM emulates some HyperV features. With the current implementation, KVM's signature is hidden, but HyperV's is not, and Nvidia drivers don't work in Windows VMs.

This blueprint generates an extra element in the libvirt xml for Windows guests on KVM which obfuscates HyperV's signature too, controlled by the existing image and flavor parameters (img_hide_hypervisor_id and hide_hypervisor_id correspondingly). The extra element is <vendor_id state='on' value='1234567890ab'/> in features/hyperv.