virsh: make completion work with older versions of virsh

Older versions of virsh doesn't have the `--name` option. Moreover when
completing running domains, the title was "name domains" instead of
expected "running domains".
virsh: replace use of compadd with _describe

compadd is more low-level and doesn't setup completion descriptions
correctly. Notably, the description is not put in bold.
virsh: inactive domains don't have an ID
Merge pull request #257 from vincentbernat/fix/virsh

Various virsh fixes