vsh: Refactor logic in vshCommandParse

Refactor the existing logic using two nested loops with a jump into the
middle of both with 3 separate places fetching next token to a single
loop using a state machine with one centralized place to fetch next
tokens and add explanation comments.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>