virsh-completer: Separate comma list construction into a function

virsh-completer: Separate comma list construction into a function

There are more arguments than 'shutdown --mode' that accept a
list of strings separated by commas. 'nodedev-list --cap' is one
of them. To avoid duplicating code, let's separate interesting
bits of virshDomainShutdownModeCompleter() into a function that
can then be reused.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>