Fix Vagrant compatibility issue: cannot build after vm:up
This branch's main purpose is to fix a compatibility issue between "old"
and "new" Vagrant reported on tails-dev@ a while ago  that prevents
doing something like:
rake vm:up rake build
The 'build' task doesn't work if issued when the VM is already up,
because of VM API changes, so a "compabitility layer" is needed.
However, it works when a `build` is issued when the VM is not up.
Since the code that fails in the former case is the cpu and memory
checks, it suggests that they are not done in the latter, which is
wrong. This branch also fixes that, so the checks are always performed,
no matter the VM state.