fix: Dynamically create netdev arguments to correctly include commas

docs: Add entrance to the changelog about the fix

Update changelog/changes/2025-01-15-qemu-startup-script-comma-fix.md

Co-authored-by: Mathieu Tortuyaux <mathieu.tortuyaux@gmail.com>

Author: John15321

build_library/qemu_template.sh

@@ -302,7 +302,7 @@ case "${VM_BOARD}" in
         qemu-system-x86_64 \
             -name "$VM_NAME" \
             -m ${VM_MEMORY} \
-            -netdev user,id=eth0,${QEMU_FORWARDED_PORTS},hostfwd=tcp::"${SSH_PORT}"-:22,hostname="${VM_NAME}" \
+            -netdev user,id=eth0${QEMU_FORWARDED_PORTS:+,}${QEMU_FORWARDED_PORTS},hostfwd=tcp::"${SSH_PORT}"-:22,hostname="${VM_NAME}" \
             -device virtio-net-pci,netdev=eth0 \
             -object rng-random,filename=/dev/urandom,id=rng0 -device virtio-rng-pci,rng=rng0 \
             "$@"
@@ -311,7 +311,7 @@ case "${VM_BOARD}" in
         qemu-system-aarch64 \
             -name "$VM_NAME" \
             -m ${VM_MEMORY} \
-            -netdev user,id=eth0,${QEMU_FORWARDED_PORTS},hostfwd=tcp::"${SSH_PORT}"-:22,hostname="${VM_NAME}" \
+            -netdev user,id=eth0${QEMU_FORWARDED_PORTS:+,}${QEMU_FORWARDED_PORTS},hostfwd=tcp::"${SSH_PORT}"-:22,hostname="${VM_NAME}" \
             -device virtio-net-device,netdev=eth0 \
             -object rng-random,filename=/dev/urandom,id=rng0 -device virtio-rng-pci,rng=rng0 \
             "$@"

changelog/bugfixes/2025-01-15-qemu-startup-script-comma-fix.md

@@ -0,0 +1 @@
+- Fixed creating netdev arguments to correctly include commas when no port forwards are passed ([flatcar/scripts#2581](https://github.com/flatcar/scripts/pull/2581))