dataplane: avoid reentrancy during virtio_blk_data_plane_stop()

When dataplane is stopping, the s->vdev->binding->set_host_notifier(...,
false) call can invoke the virtqueue handler if an ioeventfd
notification is pending.  This causes hw/virtio-blk.c to invoke
virtio_blk_data_plane_start() before virtio_blk_data_plane_stop()
returns!

The result is that we try to restart dataplane while trying to stop it
and the following assertion is raised:

  msix_set_mask_notifier: Assertion `!dev->msix_mask_notifier' failed.

Although the code was intended to prevent this scenario, the s->started
boolean isn't enough.  Add s->stopping so that we can postpone clearing
s->started until we've completely stopped dataplane.

This way, virtqueue handler calls during virtio_blk_data_plane_stop()
are ignored.  When dataplane is legitimately started again later we
already self-kick ourselves to resume processing.

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