enable selection of device based on the bus id using @<bus>

This was a key feature of the openni_camera drivers which allowed you to have multiple cameras, each on their own bus, and always get the one you wanted.

This is compatible with openni_camera, with the form "bus@index" where bus is the USB bus id (typically starts at 1) and index is the device on that bus (starts at 1, but 0 basically says "any device" and returns the first one found by the manager).

I've tested this with Asus Xtion Pro Live, 1.082 and 1.09 sensors in several configurations.