hw/usb: Add U2F key emulated mode

hw/usb: Add U2F key emulated mode

This patch adds the U2F key emulated mode.

The emulated mode consists of completely emulating the behavior of a
U2F device through software part. Libu2f-emu is used for that.

The emulated mode is associated with a device inheriting from
u2f-key base.

To work, an emulated U2F device must have differents elements which
can be given in different ways. This is detailed in docs/u2f.txt.

The Ephemeral one is the simplest way to configure, it lets the device
generate all the elements it needs for a single use of the lifetime
of the device:

    qemu -usb -device u2f-emulated

For more information about libu2f-emu see this page:
https://github.com/MattGorko/libu2f-emu.

Signed-off-by: César Belley <cesar.belley@lse.epita.fr>
Message-id: 20200826114209.28821-7-cesar.belley@lse.epita.fr
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>