Automatically collects cpu & memory & network usage stats for each VM
every minute and appends it to `/vm/logs/$vmname.log`.
An example row in that log file looks like:
```
{"cpu_usage_usec":114769860,"memory_total_kb":8128676,
"memory_free_kb":6120912,"rx_bytes":313094089,
"tx_bytes":4107151,"timestamp":"2025-01-23T04:58:13Z"}
```
We will most likely revise where we store the stats. This is just a
prototype to try different pieces end-to-end. What & how we collect
stats is also subject to change.
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