VM backends grew too much logic

[Saviq]

Too much logic has grown into our platform backends, resulting in behaviours that should be common across all platforms requiring virtually duplicate code in each of them.

We should refactor it in a way that HV backends are just interfaces to the hypervisor in question, with little to no logic at all.

This could maybe be approached with base VM classes or the daemon taking over the logic. For features unsupported on a given platform, CanonicalLtd/multipass#477 / CanonicalLtd/multipass#478 are relevant.

• [x] start() Addressed in #2611
• [ ] stop()/shutdown()
• [ ] suspend()
• [ ] on_shutdown()?
• [ ] dtor