Hi Gianluca,
I see the numa mode select box in the Host sub-tab of the VM edit dialog.
It was committed three days ago to the 3.5 branch so it should be available in the 3.5 GA release I think. I am not exactly sure if it made the RC3 build though.
http://gerrit.ovirt.org/#/q/I05b83028b722088e39ad7156c77ad4eb479dc241,n,z