remoteproc: virtio: Anchor vring life cycle in vdev

Instead of having the vrings being allocated and freed as they are
requested by the virtio device tie their life cycle to the vdev
resource. This allows us to decouple the vdev resource management from
the virtio device management.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
(cherry picked from commit a863af5d)