qemu -readconfig/-writeconfig cannot handle quotes in values

$ qemu-system-x86_64 -drive file=/tmp/foo\" -writeconfig -
# qemu config file

[drive]
  file = "/tmp/foo""

For bonus points, try to construct a value qemu config file that contains a quoted value.  It's pretty clear (from looking at the code also) that this is not possible.

Also:

- maximum value length is hard-coded in the parser at 1023 characters (for no apparent reason)

- the format is undocumented

- don't use sscanf for parsing!