Enhancement: Add domain title property

[NamelessOne91]

Related to #1041

Adds basic support for managing a libvirt domain title property.

Possible further improvements:

• have a CustomDiff function that checks if a new title is valid (read: contains no newline characters) BEFORE attempting to replace a domain. Right now when the new title contains newlines the user can still attempt a replacement, the previous domain is destroyed and the new one fails to be created because the title is invalid.
  No bueno.

• do not force a replacement when the title changes and correctly update it in place. I think libvirt API has no function to update only the title property, so this may require directly updating the domain XML configuration and redefining the domain.

[dmacvicar]

ℹ️ ℹ️ ℹ️ ℹ️ ℹ️ ℹ️

This contribution is relevant to the legacy version of the provider, which is now in the v0.8 branch of this repository.

Future development is based on a new provider, which is not compatible with this one, nor does share code.

As the new provider solves many issues with the legacy, and to make development in my free time more enjoyable, I have decided to close all PRs for the legacy provider, and to ask to check if the contribution would apply to the new one. This also to encourage trying the new version.

• https://registry.terraform.io/providers/dmacvicar/libvirt/latest

and check the documentation:

• https://github.com/dmacvicar/terraform-provider-libvirt/blob/main/README.md
• https://registry.terraform.io/providers/dmacvicar/libvirt/latest/docs

You are free to reopen the PR for v0.8, which is targetted now to the v0.8 branch. We may also start a discussion if we can assemble a team of maintainters for the legacy branch. My efforts will go into the new provider.

I ask you to check the new provider and re-evaluate this contribution. 🙏