Add a virtual VPU driver to aid userspace in testing stateless uAPI
implementations for which no hardware is currently available.

A userspace implementation can use vivpu to run a decoding loop even
when no hardware is available or when the kernel uAPI for the codec
has not been upstreamed yet. This can reveal bugs at an early stage.

Also makes it possible to work on the kernel uAPI for a codec and
a corresponding userspace implementation at the same time.

Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>