want better dev experience for cleaning up VMM reservoir allocation

[jordanhendricks]

For developers running a real sled agent, by default, sled agent will provision a VMM reservoir. This will claim some amount of memory on the machine for guests, and have other system effects related to claiming more memory (such as your system likely needing a swap device to be usable).

It would be nice if we had a good way to indicate to devs that they should consider reclaiming this memory after they are done running omicron. This can be done with the rsrvrctl tool today, but we could also consider doing it automatically as a part of the omicron uninstall tool, or a similarly automated solution.

[jordanhendricks]

As a part of fixing this, we should probably delete tools/delete-reservoir.sh in favor of using the ioctl directly, e.g., with the existing illumos-utils utility functions:

https://github.com/oxidecomputer/omicron/blob/main/illumos-utils/src/vmm_reservoir.rs