Better handling of errors while creating a domain
Better handling of errors while creating a domain

Sometimes if an error occurs at domain creation time, the resulting
domain is **unknown** to terraform. That makes impossible to remove
it, causing later runs of `terraform apply` to fail.

This commit marks the domain ID as a partial attribute that has to be
saved to the `.tfstate` as soon as it's created.

That should fix issues like #67

dmacvicar/terraform-provider-libvirt,deasmi/terraform-provider-libvirt,flavio/terraform-provider-libvirt,deasmi/terraform-provider-libvirt,dmacvicar/terraform-provider-libvirt,dmacvicar/terraform-provider-libvirt,deasmi/terraform-provider-libvirt