vsh: Deduplicate filtering in vshReadlineOptionsGenerator()

vsh: Deduplicate filtering in vshReadlineOptionsGenerator()

Completer callbacks generate all possible outputs ignoring any partial
input (e.g. prefix of a domain name) and then use vshCompleterFilter() to
filter out those strings which don't fit the partial input (prefix).

In contrast, vshReadlineOptionsGenerator() does some internal filtering and
only generates completions that match a given prefix. Rather than treating
these scenarios differently, simply generate all possible options and
filter them all at the end.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>