This patch is to add support for vhost input device emulation.
The code skeleton heavily reuses vhost-device-i2c and vhost-device-rng,
including sharing the same license, and how to create the multi-thread
mode and daemon for backend virtual device.
It gives detailed usage for enabling virtual input device for guest in
the README.md.
Signed-off-by: Leo Yan <leo.yan@linaro.org>
