Prevent creating something with an invalid name

[obbcth]

We should block creating images with an invalid name like "abc/def.img".

Also happens on creating VMs.

[osy]

Error checks and input validation in config setting in general is needed.

[morrowa]

This doesn't appear to be fixed. I tried to name a VM "macOS w/Nix" and it failed to create saying it couldn't find Nix.utm.

[osy]

It might be reintroduced in a new change. Can you make a new issue?

[morrowa]

Filed as #4385
Thanks!