qemu -readconfig/-writeconfig cannot handle quotes in values

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1686364
Reported by 'Richard Jones' on 2017-04-26 :

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

[drive]
  file = "/tmp/foo""

For bonus points, try to construct a valid 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!