Defaults are applied so recreate-unpersisted run on old prep.json
without crashing. This patch does apply any of the parameters to the
systemd units, though, that's next.
Add the following parameters to `VmSetup`'s methods:
* `slice_name`: cgroup slice name the VM should be a member of. Default
is `system.slice`, which will be used by default until we migrate to a
system with stricter resource management. Note that this works also on
Ubuntu 22.04.
* `cpu_percent_limit`: cpu quota the VM can use.
* `cpu_burst_percent_limit`: additional quota that a VM can burst into
temporarily if it has build up enough credit.
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