run: Fix wrong param assignment (nic_type/nic_model)

Paolo spotted another mistake I've made - in fact when we
select an nic model that is not in one of our variants,
the correct param name is nic_model.

CC: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>