Code review comment for ~philroche/simplestreams/+git/simplestreams:feture/filter-items-before-filter-versions

An example query where the problem this commit fixes is

```
sstream-query --max=1 -U http://cloud-images.ubuntu.com/releases/streams/v1/com.ubuntu.cloud:released:aws-cn.sjson crsn=cn-north-1 release=xenial arch=amd64 virt=hvm root_store=ssd --output-format="16.04 %(version_name)s %(arch)s %(id)s %(root_store)s %(virt)s"
```

This would return no results despite there being 41 images that satisfy those filters.