Add test coverage for crash caused by unknown values for `mac_address` argument

[ddelnano]

#232 describes a previous bug where unknown values provided to a xenorchestra_vm.network[0].mac_address argument would cause a provider crash. The reproduction was accomplished by using the popular ivoronin/macaddress terraform provider. It's possible to use these in a terraform provider acceptance test, but it the upstream provider has an issue resulting in it being unable to be used as a

There is also ambiguity on what the project's license is. This issue tracks following up with upstream on ivoronin/terraform-provider-macaddress#3 in addition to asking them to add a license.

[ddelnano]

I've opened ivoronin/terraform-provider-macaddress#4 to get clarity on the licensing for the macaddress project.