KVM: selftests: Add PTE helper for x86-64 in preparation for hugepages

Add a helper to retrieve a PTE pointer given a PFN, address, and level
in preparation for adding hugepage support.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622200529.3650424-17-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>