GUI: client should get prefix of all machines on instlist

As a developer I would like the GUI to correctly get the prefix for a machine i'm switching instrument to, if the instrument is not shown on the dropdown list of instruments (which i think are just the "scheduled" instruments in `CS:INSTLIST`).

For example, typing `NDXHIFI-CRYOMAG` should give the PV prefix of `IN:HIFI-C11:` but it leaves the prefix text box blank currently.

Currently this only works for "scheduled" instruments. It opens a modal box which asks the user to enter the PV prefix, but I think we should add a default value for this textbox.

It would also be nice if it worked for machines NOT on the instlist, ie. putting NDXXYYYY defaults to a prefix of TE:NDXXYYYY:

### Acceptance criteria
- [ ] Client puts in prefixes for ALL instruments that are in the `CS:INSTLIST` PV
- [ ] Client guesses with TE:NDXXYYYY for other machines