I tried it in 32-bit WinXP in a VirtualBox VM, running on a 64-bit Linux Mint (Ubuntu-based debian distro).
That environment works for MfgTool, so should also work for fastboot.
I have libusb-1.0 drivers installed in Windows.
EDIT: I previously tried your fastboot from a few days ago while trying different versions of libusb-win32 installed by different methods, none of which could see my kindle in fastboot mode even when selected in Virtualbox USB devices. I am also having some "missing headers" issues when I try to compile your source code, using MinGW.
This debug version reports "busses count: 1" "< waiting for device >".