KVM: X86: Fix when shadow_root_level=5 && guest root_level<4

If the is an L1 with nNPT in 32bit, the shadow walk starts with
pae_root.

Fixes: a717a780 ("KVM: x86/mmu: Support shadowing NPT when 5-level paging is enabled in host)
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Message-Id: <20211124122055.64424-2-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>