virtio_ring: make vring_alloc_queue_packed prettier

Add some spaces to vring_alloc_queue(make it look prettier).

Signed-off-by: Deming Wang <wangdeming@inspur.com>
Message-Id: <20220926183306.4535-1-wangdeming@inspur.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>