if you enable the "agent" option on the VM, then PVE will automatically generate a serial device for the agent:
-chardev 'socket,path=/var/run/qemu-server/<VMID>.qga,server=on,wait=off,id=qga0' \
-device 'virtio-serial,id=qga0,bus=pci.0,addr=0x8' \
-device...