Modify and fix various --*-machine options (and break a few)

`get_info_for_machine' now works without client configuration.

This involved replacing `is_client_up' calls with `is_vbox_up', for
now. Later, we need a `is_machine_up' function, or similar.

A limitation in this patch is that it breaks Android machines (not
Android clients), and possibly also QEMU machines. This will most
likely be fixed early next week.

A client configuration is now optional in several places, most notably
in `stop_helper'.