assistant: display confirmation dialog if no OS is selected

Make it clearer to users, through a confirmation dialog, that their experience may be degraded if they install a VM without selecting an operating system.

Only display this dialog if no operating is selected.

Without OS	With OS
demo_with_mint	demo_with_fedora

The message that gets displayed is:

Creating a box without selecting an operating system may significantly worsen your experience. Would you like to proceed anyway?

But I can change it, if someone comes up with a clearer way of saying it it :)