server: add a first-class error type to machine init

Add a `MachineInitError` type and make `MachineInitializer` functions
return it. This makes it more convenient to handle new logic errors in
initializer functions. To handle the (many) cases where the initializer
calls bhyve ioctls or filesystem functions that return `std::io:Error`,
`MachineInitError` has a catchall `anyhow::Error` variant; this
encourages calls that produce I/O errors to call `context` or
`with_context` to better describe those errors.

Tested by running propolis-server ad hoc in a couple of invalid
configurations (file backend not pointing to an actual file; too many
vCPUs; more guest memory than the host has available) and verified that
the error chains (and contexts, if applicable) show up in the Propolis
logs.