[cli] Tab completion for multipass: do not list generic options

**Describe the bug**
When using tab completion with the multipass CLI and bash, --verbose and --help are listed whenever a completion is asked for. This creates noise that makes it harder to find the relevant information.

**To Reproduce**
How, and what happened?
1. `multipass start `
1. --all --verbose logical-condor useful-apaca --verbose

**Expected behavior**
--all logical-condor useful-apaca

Start by reproducing the reported Bash completion with `multipass start ` and compare the current suggestions with the expected list. Find the Multipass CLI completion entry point that supplies these options, then verify that `--verbose` and `--help` are no longer listed while command-specific options and instance names remain available.

Written by the indexing model from the issue text.