Feature/Virtual device support for Midi services

[NeoCortex97]

Description

I think the midi service should provide a setting so it can present a virtual input or output.

Virtual Midi ports are not reflected in hardware and are made to allow communication between two midi capable softwares.
I think both inputs and outputs should be supported.

[carterfyi]

I may not be understanding your specific feature request, but at first glance this seems duplicative of the existing MIDI service implementation. Virtual midi device instantiation on-the-fly would be pretty heavily OS dependent, and there are existing solutions for this by platform. On Windows, for example, I've used loopMIDI for years, which could create and expose the virtual ports to be used by the nodecg-io MIDI service. Unless there's something I'm missing, it seems this feature might be out of scope for the project?