virsh: Change option parsing functions to return tri-state information

virsh: Change option parsing functions to return tri-state information

This is needed to detect situations when optional argument was
specified with non-integer value: '--int-opt foo'. To keep functions
uniform vshCommandOptString function was also changed, because it
returns tri-state value as well. Given result pointer is updated only
in case of success. If parsing fails, result is not updated at all.