Enable Webcam passthrough

[tcr]

Webcam control over USB passthrough has many issues in virtualbox: https://www.virtualbox.org/ticket/242

4.3 has experimental support for webcam specific passthrough: https://www.virtualbox.org/manual/ch09.html#idp97292096

If we support USB passthrough configuration (see #9), webcam passthrough can be seamless, e.g.

$ VBoxManage list webcams
Video Input Devices: 2
.1 "Live! Cam Sync HD VF0770"
0x1a120000041e4095
.2 "FaceTime HD Camera (Built-in)"
0x1a11000005ac8510

The last digits of the address (0x1a110000 / 05ac / 8510) match the vendor and device IDs, which is what USB devices in general would be matched on. So, after boot:

$ VBoxManage controlvm tessel2 webcam attach .1

[clockzhong]

I've tested the USB passthrough in Virtualbox 5.2.8 in Ubuntu16.04 Host, it still has serious problem.