Support mod devices CV port

[ctsexton]

Mod devices define their own sub class of CVPort here: https://moddevices.github.io/mod-sdk/mod/#CVPort

This makes it possible to load plugins like Cardinal that define the mod CVPort. That plugin actually lists both the lv2:CVPort and mod:CVPort in the manifest file:

    [
        a lv2:InputPort, lv2:CVPort, mod:CVPort ;
        lv2:index 8 ;
        lv2:symbol "lv2_cv_in_1" ;
        lv2:name "CV Input 1" ;
        lv2:minimum 0.0 ;
        lv2:maximum 10.0 ;
        lv2:portProperty lv2:connectionOptional;
    ]

Perhaps there is an alternative way to accept plugins like this since at least one of the port properties matches the accepted port types? I imagine there could be many more plugins like this that define their own port subclasses but which are entirely compatible with livi.