Add search query in Web UI to look for VMs by cluster compatibility level override (custom_compatibility_version field in "vms" view).

Additional info:

In draft implementation, when we write a search query:

Vms: custom_compatibility_level = 4.1

it is translated into SQL:

select ... where custom_compatibility_version IS NULL OR custom_compatibility_version = '4.1';

which is not what we want.

This is a standard translation for all string expressions, so we need to fix the parser to generate a correct SQL just in this case:

select ... where custom_compatibility_version = '4.1';