virsh: completer: Introduce dummy completer for local files

virsh: completer: Introduce dummy completer for local files

For now the completion does the correct thing of completing a local path
if NULL is returned.

Introduce 'virshCompletePathLocalExisting' and use it in the
'VIRSH_COMMON_OPT_FILE' macro.

This for now serves as an annotation for the function which want to read
a file on the host running virsh. In the future this can be used with a
more sophisticated implementation.

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