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: NHou Tao <houtao1@huawei.com>
Reviewed-by: NStefano Garzarella <sgarzare@redhat.com>
Reviewed-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>