virsh: Introduce virshCompleteEmpty and use it for places where we can't suggest...

virsh: Introduce virshCompleteEmpty and use it for places where we can't suggest anything

For now this serves just as an annotation because readline and also the
bash completion script insist on completing local paths when an empty
list is returned.

This will serve for future reference once we'll be able to properly
refuse to suggest anything.

The completer is used for fields such as names for new objects,
description strings, password strings etc, URIs and hostnames which we
can't feasibly autocomplete.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>