vsh: Call vshCmdOptDef completer

vsh: Call vshCmdOptDef completer

Now that we have everything prepared we can call options'
completer again. At the same time, pass partially parsed input to
the completer callback - it will help the callbacks to narrow
down the list of returned options based on user's input. For
instance, if the completer is supposed to return list of
interfaces depending on user input it may return just those
interfaces defined for already specified domain. Of course,
completers might ignore this parameter.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>