KVM: MMU: check kvm_mmu_pages and mmu_page_path indices

Give a special invalid index to the root of the walk, so that we
can check the consistency of kvm_mmu_pages and mmu_page_path.

Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
[Extracted from a bigger patch proposed by Guangrong. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>