kvm_mmu_free_roots() can not free those new types of sp if they are still
valid. And different vcpu can use the same pae root sp if the guest cr3
of the vcpus are the same.
And new pae root can be put in prev_root too (not implemented yet)
because they are not too special anymore. As long as sp->gfn, sp->pae_off,
sp->role are matched, they can be reused.