Description of problem:
When the user tries to connect in kiosk mode to a server that requires password authentication, there's no way to cancel the authentication process and close the application. Pressing the "Cancel" button or trying to close the auth dialog doesn't work, the dialog reappears.

This might be desired behavior given what the documentation says about kiosk mode. But because virt-viewer doesn't grab the keyboard before it's connected, you can still escape (e.g. by pressing Alt+Tab and killing it manually). So as a result, it only causes unnecessary inconvenience -- for example when you realize that you're connecting to a wrong machine you don't know the password to.

On the other hand, I can imagine that it can be undesirable to allow users to end the authentication, for example:
you setup the connection -> the remote machine later restarts -> authentication must be performed again, but now you don't want others to close it, because that would mean escaping.

How reproducible:
100%

Steps to Reproduce:
1. prepare a vm that requires password authentication
2. connect with virt-viewer in kiosk mode
$ virt-viewer -k -c ...
3. try to close the dialog or press "Cancel"

Actual results:
Authentication cannot be cancelled.

Expected results:
Authentication can be cancelled when trying to connect for the first time.

Additional info: