Confusing error on transfer argument with wrong format

Originally reported on [discourse](https://discourse.ubuntu.com/t/cant-make-transfer-work-using-home-on-windows/23957).

Multiple colons in transfer arguments result in a confusing error:
```
PS > $HOME
C:\Users\some_user
PS > multipass transfer -vvvv asdf beefy-constrictor:$HOME/asdf
An instance name is needed for either source or destination
```

Reproduce the shown `multipass transfer` command with a path containing multiple colons, then trace the transfer argument parsing and error reporting entry points. Done means the malformed argument produces a clear, actionable error instead of saying that an instance name is needed.

Written by the indexing model from the issue text.