vshReadlineOptionsGenerator: Don't add already specified options to the list

vshReadlineOptionsGenerator: Don't add already specified options to the list

The current state of art is as follows:

 1) vshReadlineOptionsGenerator() generate all possible --options
 for given command, and then
 2) vshReadlineOptionsPrune() clears out already provided ones
 from the list.

Not only this brings needless memory complexity it is also not
trivial to get right. We can switch to easier approach: just
don't add already specified --options in the first step.

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