virtio-balloon: unref the iothread when unrealizing

virtio-balloon: unref the iothread when unrealizing

We took a reference when realizing, so let's drop that reference when
unrealizing.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Fixes: c13c4153f76d ("virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT")
Cc: qemu-stable@nongnu.org
Cc: Wei Wang <wei.w.wang@intel.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200520100439.19872-4-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>