Else there will be memory leak if alloc_disk() fails.

Fixes: 6a27b656 ("block: virtio-blk: support multi virt queues per virtio-blk device")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>