Coincidentally I did exactly the same yesterday (deploying Fedora Rawhide within
Oracle's Virtualbox 5.1.14 r112924) and obtained the very same result. After
installation and reboot the SDDM login screen shows up nicely, but once I log in I get the
blank screen. The mouse is there, it responds to movement as expected but absolutely
nothing more! Doesn't seem to be a graphical issue. Any word from the dev team would
be very welcome!