QemuOpts: framework for storing and parsing options.

This stores device parameters in a better way than unparsed strings.

New types:
  QemuOpt       -  one key-value pair.
  QemuOpts      -  group of key-value pairs, belonging to one
                   device, i.e. one drive.
  QemuOptsList  -  list of some kind of devices, i.e. all drives.

Functions are provided to work with these types.  The plan is that some
day we will pass around QemuOpts pointers instead of strings filled with
"key1=value1,key2=value2".

Check out the next patch to see all this in action ;)

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
