[libvirt] Need help for thesis on Xen - disabling HAP for benchmarking purposes

Hi, I'm a German student, writing my thesis on "Virtualization with Xen. Analysis and Comparision of Different Techniques such as Paravirtualization, Full Virtualization, and Utilization of Hardware Support Provided by the Processor". I'd like to know how I can disable HAP/RVI/Nested Paging for benchmarking purposes. I read somewhere (http://markmail.org/message/bbnivuqx6vjz7jg4) that Xen developers decided to disable the global grub parameter introducing a per domain flag for HAP instead (hap=0/1). I used virt-manager built in CentOS for setting up all domUs; virt-manager uses libvirt so that no "old-style" configuration files are stored in /etc/xen. I actually don't know how if libvirt-XML-files already support something like <hap>0</hap>. How would you proceed to modify an already existing domain using libvirt/virt-manager? I really do apologize for my "newbie ignorance". Thanks in advance for any useful help. - Alex