virtio-pmem: do delete rq_vq in virtio_pmem_unrealize

virtio-pmem: do delete rq_vq in virtio_pmem_unrealize

Similar to other virtio-devices, rq_vq forgot to delete in
virtio_pmem_unrealize, this patch fix it.  This device has already
maintained a vq pointer, thus we use the new virtio_delete_queue
function directly to do the cleanup.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Message-Id: <20200225075554.10835-4-pannengyuan@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>