Bash completion suggests multiple instances for single-instance commands

```
$ multipass shell [TAB][TAB]
aspirant-ermine --help luscious-elf --verbose
$ multipass shell aspirant-ermine [TAB]
--help luscious-elf --verbose
```
But shell only accepts one instance... Other commands are affected too.

**Expected behavior**
No instances would be suggested after the first free argument. In `shell` or `snapshot`, free arguments could be identified by whether or not they start with a dash. In `exec` or `mount`, it is a little trickier.

Start by tracing the Bash completion behavior for the shell, snapshot, exec, and mount commands, then reproduce the examples with multipass shell. Done means completion stops suggesting instance names after the first free argument while preserving appropriate options and handling the trickier exec and mount cases.

Written by the indexing model from the issue text.