wizard: Instantiate prepare_cancellable only once

It is useless to reinstantiate it several times. This also lowers
code complexity a bit.

https://bugzilla.gnome.org/show_bug.cgi?id=725386
installer-media: prepare() return true on success

This allows callers to handle cancellation/failure.

https://bugzilla.gnome.org/show_bug.cgi?id=725386
wizard: Handle return value of prepare()

This way if InstallerMedia.prepare is cancelled, it returns false and
Wizard stops continuing with rest of preparation.

This fixes the issue of Wizard jumping to setup page after driver
download is finished even if user has cancelled their request to create
the corresponding VM.

https://bugzilla.gnome.org/show_bug.cgi?id=725386