vsh: Allow double quotes imbalance for auto completion in vshCommandStringGetArg()

vsh: Allow double quotes imbalance for auto completion in vshCommandStringGetArg()

If user is trying to auto complete a value that contains a space,
they have two options: use backslash to escape space or use
quotes, like this:

  virsh # start --domain "domain with space<TAB>

However, in this case our tokenizer sees imbalance in (double)
quotes: there is a starting one that's missing its companion.
Well, that's obvious - user is still in process of writing the
command. What we need to do in this case is to ignore the
imbalance and return success (from the tokenizer) - readline will
handle closing the quote properly.

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