This keeps information about vendors and USB IDs in one place, and
allows us to extend with model-specific information later.
By default, this should be backwards-compatible -- TrezorClient can
optionally accept model information, and if not, it will try to guess
based on Features.
It is possible to specify which models to look for in transport
enumeration. Bridge and UDP transports ignore the parameter, because
they can't know what model is on the other side.
supersedes #1448 and #1449
