hw/usb: Add U2F key base class implementation

hw/usb: Add U2F key base class implementation

This patch adds the U2F key base class implementation.

The U2F key base mainly takes care of the HID interfacing with guest.
On the one hand, it retrieves the guest U2FHID packets and transmits
them to the variant associated according to the mode: pass-through
or emulated.
On the other hand, it provides the public API used by its variants to
send U2FHID packets to the guest.

Signed-off-by: César Belley <cesar.belley@lse.epita.fr>
Message-id: 20200826114209.28821-5-cesar.belley@lse.epita.fr
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>