x86/access: Stash root page table level in test environment

Track the root page table level in the test environment, a future commit
will use it to guage whether or not the number of page tables being
allocated is reasonable.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>