x86/access: Add proper defines for hardcoded addresses

Use defines to differentiate between the paging structure and code/data
physical address ranges to reduce the magic ever so slightly.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>