I've upgraded from QEMU 2.9.0 to 2.10.0, and noticed that my USB passthrough wasn't working anymore, so I had to change the parameters. The syntax is quite confusing, so I'll document it here.
Take, for example, this nice, and simple, legacy syntax:
to change that into the new [-device usb-...] syntax, it turns into:
This really threw me off at first, because of the `0x` you have to prepend before the vendorid and productid parameters.
Hope this help for people that upgrade to 2.10.