vshCmddefGetOption: Change type of opt_index

vshCmddefGetOption: Change type of opt_index

This function tries to look up desired option for a given parsed
command. Upon successful return it also stores option position
into passed *opt_index. Now, this variable is type of int, even
though it is never ever used to store negative value. Moreover,
the variable is set from a local variable which is type of
size_t.

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