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Initial skeleton for virtio-video crate.
This crate is based on the v3 of the virtio-video
specs patch[1].

It has a big part of the infrastructure
required, although not all commands are implemented,
and does not have any backend available.

Includes support for async responses to the driver
(through VIDEO_EVENT) to QueueResource messages.

[1] -
https://lists.oasis-open.org/archives/virtio-dev/202002/msg00002.html

Related: #364
Signed-off-by: Albert Esteve <aesteve@redhat.com>