fleeting-plugin-libvirt v0.1.3

A clearer connection error. Plugin behaviour and configuration are unchanged.

- When a guest has no address, the connection error now reports the lease lookup as well as the guest agent: the lookup's error, or that it found no usable lease.

Install: `registry.gitlab.com/dunn.dev/fleeting-plugin-libvirt:0.1.3` (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:0.1.3
```

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.