Xpra: Ticket #1692: usb forwarding

We usually forward devices using an abstraction:

• audio is forwarded as streams (#246 and older)
• printers are virtualized (#598)
• input devices can be mapped to uinput virtual devices (#1611)

etc

But this doesn't work for all device types, some users may want to be able to access more esoteric devices remotely.

For that, we could use:

• usbip: a general USB device sharing system over IP network
• UsbRedir (from spice): a network protocol for sending usb device traffic over a network connection (supported in qemu)