This requires deamonization and port forwarding the ssh. I am fuzzy how this all works, but I saw the vagrant-qemu generate something like the following (forgive non-formatted at Medium doesn't support that for replies) below. Afterward, at least doing this via vagrant, one can ssh to port 50022 as the vagrant user (using the well known private key).

qemu-system-x86_64 \
-machine virt,accel=hvf,highmem=on \
-cpu host \
-smp 2 \
-m 4G \
-device virtio-net-device,netdev=net0 \
-netdev user,id=net0,hostfwd=tcp::50022-:22 \
-drive if=virtio,format=qcow2,file="$IMG_PATH" \
-chardev socket,id=mon0,path=$SOCKET_PATH,server=on,wait=off \
-mon chardev=mon0,mode=readline \
-chardev socket,id=ser0,path=$SERIAL_PATH,server=on,wait=off \
-serial chardev:ser0 \
-pidfile $PID_PATH \
-daemonize \
-parallel null \
-monitor none \
-display none \
-vga none