Serial Only

AMD64

qemu-system-x86_64 ... -nographic -append "root=/dev/vda rw console=ttyS0" -hda rootfs.img

ARM64

qemu-system-aarch64 ... -nographic -append "root=/dev/vda rw console=ttyAMA0" -hda rootfs.img

Some emulated consoles will need a speed appended like console=ttyAMA0,115200