[Qemu-block] please fix block device FIXMEs in 2.4's changelog

[Peter Maydell]

There are two lines in 2.4's changelog (http://wiki.qemu.org/ChangeLog/2.4)
in the "Block devices in system emulation" section which read:

 * FIXME: Throttle groups

and

 * FIXME: incremental backup?

Could somebody fill these in with real content before we all forget
what was and wasn't in 2.4?

thanks
-- PMM