x86/access: Refactor so called "page table pool" logic

Track the index instead of raw physical address in the page table pool
to make the "enough room" check a bit less magical.

Opportunistically append "_pa" to pt_pool to clarify that it's the base
physical address of the pool.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>