KVM: selftests: Rename x86's page table "address" to "pfn"

Rename the "address" field to "pfn" in x86's page table structs to match
reality.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622200529.3650424-16-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>