QemuOpts: command line switches for the config file.

Adds -readconfig and -writeconfig command line switches to read/write
QemuOpts from config file.

In theory you should be able to do:

  qemu < machine config cmd line switches here > -writeconfig vm.cfg
  qemu -readconfig vm.cfg

In practice it will not work.  Not all command line switches are
converted to QemuOpts, so you'll have to keep the not-yet converted ones
on the second line.  Also there might be bugs lurking which prevent even
the converted ones from working correctly.

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