domU memory exceeded =?=> spontaneous reboots

[Mike <debian@xxxxxxxxxxxxxxxxxxxxx>]

> One of the first commands that I try to execute in one
> domU, which causes activity in the other domU, triggers a reboot.
> 
> So I pinned each of the domUs' vCPUs: one with `cpus = "4-5"` and the other
> with `cpus = "all,^0-5"`.  Also reduced `vcpus` and `maxvcpus` in the latter,
> to avoid oversubscription.  Testing...

So what "solved" the issue, or at least seems to avoid it for now, is increasing
the `memory` and `maxmem` of the first domU to `4096` and the `vcpus` and
`maxvcpus` to `3`.  I probably didn't even need to increase the vCPUs; I think
it's a memory constraint issue.

The first domU is the sole control node for a Kubernetes cluster.  It also has
2GB of swap available to it, but I'm not sure that k8s will use it, since it
seems to avoid swap for sensitive data.  The minimum suggested for k8s is two
CPUs and 2GB of memory, which is what I initially chose.

The second domU is the sole worker node for the cluster.  The command that I
ran in it that triggered the reboot was `kubectl delete -f` of a Deployment
that was already running from an `apply`.

A `free -m` on the control node domU outputs:

---
      total  used  free  shared  buff/cache  available
Mem:   3913  1391   856       2        1894       2521
Swap:  2047     0  2047
---

This seems likely to be a Xen bug.  An attempt to overuse memory from inside
a domU shouldn't reboot the hardware.