sudo apt-get install cloud-image-utils

cat >user-data <<EOF
#cloud-config
password: a
chpasswd: { expire: False }
ssh_pwauth: True
EOF

cloud-localds user-data.img user-data

# user-data.img MUST come after the rootfs. 
qemu-system-x86_64 \
-drive file=ubuntu-18.04-server-cloudimg-amd64.img,format=qcow2 \
-drive file=user-data.img,format=raw