fleeting-plugin-libvirt v0.1.2

Version reporting, a security update and documentation; no change to plugin behaviour or configuration.

- Reports its version and build revision to the host, so gitlab-runner and forgehand log them at "plugin initialized" (fleeting updated to the release that passes them through).
- Updates go-ntlmssp to v0.1.1 (security).
- README: a Support section states the support level and the 0.x configuration policy.

Install: `registry.gitlab.com/dunn.dev/fleeting-plugin-libvirt:0.1.2` (aliases `0.1`, `0`, `latest`).

Verify:

```
cosign verify \
  --certificate-identity-regexp '^https://gitlab\.com/dunn\.dev/fleeting-plugin-libvirt//\.gitlab-ci\.yml@refs/tags/.+$' \
  --certificate-oidc-issuer https://gitlab.com \
  registry.gitlab.com/dunn.dev/fleeting-plugin-libvirt:0.1.2
```

Before 1.0, a minor release (0.2) may rename, remove or change a `plugin_config` key; a patch release (0.1.x) never does. Each break is listed in that release's notes with the change to make.