https://www.kernel.org/doc/Documentation/virtual/kvm/nested-vmx.txt
Known limitations
The current code supports running Linux guests under KVM guests.
Only 64-bit guest hypervisors are supported.
Thats why even when its enabled on host and the instruction for it is visible in the guest, it still doesn’t work. Can you please make kvm builds 64-bit?
For those who want to test make sure you detect nested capability on the host first. By now Linux enables them from out of the box.
http://ebalaskas.gr/wiki/kvm/nested