Run the emulation with:
  
   qemu-system-ppc -M virtex-ml507 -kernel output/images/vmlinux -m 256 -nographic -append "console=ttyS0" -dtb output/images/virtex440-ml507.dtb
  
  The login prompt will appear in the terminal that started Qemu.
  
  Tested with QEMU 2.5.0