enable XCP-ng nested virtualisation

Enable XCP-ng nested virtualisation:

used here to test proxmox under XCP-ng as host

# xe vm-param-set uuid=<uuid> platform:exp-nested-hvm=true
# xe vm-param-set uuid=<uuid> platform:nic_type="e1000"




found here :
https://github.com/xcp-ng/xcp/wiki/Testing-XCP-ng-in-Virtual-Machine-(Nested-Virtualization)