Flickering Wayland guests are a known issue:
https://bugzilla.redhat.com/show_bug.cgi?id=1266484 .
Despite this you can still navigate with the keyboard and thus log in. If you chose the
GNOME on Xorg session, then you get a usable desktop. You can also disable Wayland for GDM
altogether in /etc/gdm/custom.conf.