Run the emulation with:
qemu-system-x86_64 -M pc -kernel output/images/bzImage -drive file=output/images/rootfs.ext2,if=virtio,format=raw -append "root=/dev/vda" -net nic,model=virtio -net user
Optionally add -smp N to emulate a SMP system with N CPUs.
The login prompt will appear in the graphical window.
Tested with QEMU 2.9.0