Hello there,
Slackware's libusb is the problem. Slackware's libusb is too old.
First install usbredir fro Slackbuilds.org. Then google for libusb-1.0.19.tar.xz, and place it in the qemu slackbuild directory with the qemu source package.
And run 'LIBUSB=yes ./qemu.SlackBuild'. Then install the resulting package which will be placed in /tmp, with installpkg.
Good luck