Implement USB proxy sample app

[Sheph]

the idea is simple:

* take over some real device, get its description and configurations
* make a virtual device with same description and configurations
* proxy all usb requests from our virtual device to this real device

run the proxy with some complicated devices such as USB cameras, it'll help
us test existing libvdpusb functionality and implement missing pieces, after
that we can move on to more complicated things such as usb<->ip

[Sheph]

simple USB sticks (i.e. those without extended descriptors) already work!

[Sheph]

extended descriptors supported, USB cameras now work! still need to support isochronous mode though so that all kinds of cameras could work...

[Sheph]

So right now vdpusb-proxy looks very solid, it can handle almost anything, what it still can't handle is isochronous transfers, will work on that...

[Sheph]

Implemented isochronous "in" transfers, got both of my isochronous USB cameras work! perfect!
But I wonder how to test isochronous "out" transfers, I don't have such devices...

[Sheph]

Implemented and tested "out" isochronous transfers, I used usb-based bluetooth controller with a headset device, that triggered "out" isochronous transfers. Thus, vdpusb-proxy is pretty much complete!