[#68445956] Add missing Virtual Server persistence schema

I'd missed adding the schema sections for VirtualServer persistence sections, as we don't use them in our vCD orgs. This meant that the (working) generator code could never be called, and effectively broke the feature.

I've added the schemas and tests to ensure they are working as expected, plus included them in our integration tests so we don't inadvertently regress them out again.

This fixes https://github.com/alphagov/vcloud-edge_gateway/issues/36