KVM: x86/mmu: do not consult levels when freeing roots

Right now, PGD caching requires a complicated dance of first computing
the MMU role and passing it to __kvm_mmu_new_pgd(), and then separately calling
kvm_init_mmu().

Part of this is due to kvm_mmu_free_roots using mmu->root_level and
mmu->shadow_root_level to distinguish whether the page table uses a single
root or 4 PAE roots.  Because kvm_init_mmu() can overwrite mmu->root_level,
kvm_mmu_free_roots() must be called before kvm_init_mmu().

However, even after kvm_init_mmu() there is a way to detect whether the
page table may hold PAE roots, as root.hpa isn't backed by a shadow when
it points at PAE roots.  Using this method results in simpler code, and
is one less obstacle in moving all calls to __kvm_mmu_new_pgd() after the
MMU has been initialized.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>