TL DR

Step 1) Add

-object memory-backend-file,id=mem,size=512M,mem-path=/dev/shm/qemu-ram,share=on -machine memory-backend=mem

To your QEMU command

Note : Choose the correct size, the same as specified by -m, here -m 512m, therefore size=512M.

Step 2) Open /dev/shm/qemu-ram from your external process

Step 3) Profit !