virtio-blk: fix "disabled data plane" mode

In disabled mode, virtio-blk dataplane seems to be enabled, but flow
actually goes through the normal virtio path.  This patch simplifies a bit
the handling of disabled mode.  In disabled mode, virtio_blk_handle_output
might be called even if s->dataplane is not NULL.

This is a bit tricky, because the current check for s->dataplane will
always trigger, causing a continuous stream of calls to
virtio_blk_data_plane_start.  Unfortunately, these calls will not
do anything.  To fix this, set the "started" flag even in disabled
mode, and skip virtio_blk_data_plane_start if the started flag is true.
The resulting changes also prepare the code for the next patch, were
virtio-blk dataplane will reuse the same virtio_blk_handle_output function
as "regular" virtio-blk.

Because struct VirtIOBlockDataPlane is opaque in virtio-blk.c, we have
to move s->dataplane->started inside struct VirtIOBlock.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
