As @jevank wrote in many topics about Windows Qubes You don’t need to install any agents in VM to be able to use USB by qrexec. You just need to
qvm-features [VM] stubdom-qrexec 1
and install appropriate drivers for emulated USB hub. In Ubuntu Live CD it looks like some driver problem. USB drive is recognized and after some operations (don’t know what happens but flashdrive blinking it’s LED and I can’t mount/unmount because of write operation in progress) it is mounted. It stops to work when I try to write something to it (or reformat it). On WIndows it works like a charm (even on XP when I found right driver for USB hub).