Thanks for this. The only things I changed in the grub.cfg:
1. I added a serial generated from here -https://xpenogen.github.io/serial_generator/index.html
2. Added mac addresses for my network devices.
I had this in my config file:
args: -device 'nec-usb-xhci,id=usb-ctl-synoboot,addr=0x18' -drive 'id=usb-drv-synoboot,file=/path/images/<VM_ID>/synoboot.img,if=none,format=raw' -device 'usb-storage,id=usb-stor synoboot,bootindex=1,removable=off,drive=usb-drv-synoboot'
but this didnt seem to work so I ended up converting the synoboot.img into qcow2 format:
qemu-img convert -f raw -O qcow2 synoboot.img synoboot.qcow2
and then added it to my config:
sata0: local:<VM_ID>/synoboot.qcow2,size=29M
So in the above I ended up using Jun's Loader v1.04b and the latest pat file - DSM_DS918+_25423.pat
I have it now installed. I just need to add the drives.