tools/virtio: virtio_test tool

This is the userspace part of the tool: it includes a bunch of stubs for
linux APIs, somewhat simular to linuxsched. This makes it possible to
recompile the ring code in userspace.

A small test example is implemented combining this with vhost_test
module.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>