Create the virtio-device which is abstract. All the virtio-device can extend
this class. It also add some functions to virtio-bus.
Signed-off-by: NKONRAD Frederic <fred.konrad@greensocs.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

Introduce virtio-bus. Refactored transport device will create a bus which
extends virtio-bus.
Signed-off-by: NKONRAD Frederic <fred.konrad@greensocs.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>