It seems that gnome-boxes requires a package called 'SPICE guest tools' to be installed on a guest system, in order for USB devices to be redirected from the host to the guest. I am running Lubuntu 18.04 as a guest in a gnome box and I am confused as to where I can download this 'guest tools' package.
The gnome-boxes properties dialog window says it can be downloaded from http://www.spice-space.org/download.html ; however, I can only find Windows binaries for the guest tools but not for Linux.