qemu -writeconfig problem

[qemu user]

Hello.
I am trying to use qemu 4.2.0 on ArchLinux.
When I try to use the -writeconfig/-readconfig options, I have a 
problem: some of the parameters specified on the command line are not 
saved in the config.
For example, I set the block device in command line options:
-blockdev driver=file,filename=file.img,node-name=disk-image
-blockdev driver=raw,file=disk-image,node-name=root-hdd
-device 
virtio-blk-pci,drive=root-hdd,physical_block_size=4096,logical_block_size=512

But the blockdev backend setting is not saved in the file, only [device] 
section:
[device]
  driver = "virtio-blk-pci"
  drive = "root-hdd"
  physical_block_size = "4096"
  logical_block_size = "512"

How can I fix the situation? For example, the [blockdev] sections I 
added manually - qemu considers it an error.

Thank you