SIE-200 (TrustZone) MPC: BLK_MAX returns an incorrect value

Version: 
$ qemu-system-arm --version
QEMU emulator version 3.0.92 (v3.1.0-rc2-31-gd522fba244)

Arm SIE-200 Technical Reference Manual describes that BLK_MAX indicates the maximum value of "block based index register" (BLK_IDX). For example, the value 1 would indicate that BLK_IDX can be 0 or 1. According to my experiments, the AN505 FPGA image apparently follows this behavior.

In the current implementation of QEMU, it appears to indicate the number of possible values for BLK_IDX, i.e., one plus the value it's supposed to return.