dataplane: sync virtio.c and vring.c virtqueue state

Load the virtio.c state into vring.c when we start dataplane mode and
vice versa when stopping dataplane mode.  This patch makes it possible
to start and stop dataplane any time while the guest is running.

This will eventually allow us to go back to QEMU main loop for
bdrv_drain_all() and live migration.  In the meantime, this patch makes
the dataplane lifecycle more robust but should make no visible
difference.  It may be useful in the virtio-net dataplane effort.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
