use qemu-agent variable as terraform variable for domain

[MalloZup]

fix #374

[MalloZup]

for sumaform the behaviour is invisible for users, see moio/sumaform@740c5ab

[moio]

Note also that it is possible to emulate the old behavior since Terraform allows to set variables via environment variables, in case there is a actual need for those:

https://www.terraform.io/docs/configuration/environment-variables.html#tf_var_name

[MalloZup]

@dmacvicar feel free to review it, tia 🌻

[dmacvicar]

I like this approach more than the old one. It is a breaking change, but hey, we are still 0.X