hw/usb: Add U2F key base class

hw/usb: Add U2F key base class

This patch adds the specification for the U2F key base class.
Used to group the common characteristics, this device class will be
inherited by its two variants, corresponding to the two modes:
passthrough and emulated

This prepares the U2F devices hierarchy which is as follow:
USB device -> u2f-key -> {u2f-passthru, u2f-emulated}.

Signed-off-by: César Belley <cesar.belley@lse.epita.fr>
Message-id: 20200826114209.28821-4-cesar.belley@lse.epita.fr
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>