wizard: go to setup step after prepare
wizard: go to setup step after prepare

During prepare, the source/media is recognized, and then
it goes to SETUP. If the media is already known, still go
to SETUP and not REVIEW step. Also get rid of Idle().

This get rid of extra warning when selecting an iso and
staying in prepare step.

https://bugzilla.gnome.org/show_bug.cgi?id=677117

vbenes/gnome-boxes,vbenes/gnome-boxes