XMLs are stored in compressed format (as Qt resource) inside the FIT
binary. You can find one approach of extracting them in Positive
Technologies blog:

http://blog.ptsecurity.com/2017/04/intel-me-way-of-static-analysis.html

You can indeed use FIT for setting the strap but you'll have to find it yourself :) Or you
can just edit the descriptor directly - it's not protected by any
checksums or signatures. Check e.g. how me_cleaner sets the HAP bit:

    print("Setting the HAP bit in PCHSTRP0 to disable Intel ME...")
    fdf.seek(fpsba)
    pchstrp0 = unpack("<I", fdf.read(4))[0]
    pchstrp0 |= (1 << 16)
    fdf.write_to(fpsba, pack("<I", pchstrp0))