QEmu basically provides(emulates) the central processing binaries through dynamic binary translation and provides a set of device models, enabling it to run a variety of unmodified guest operating systems. Means if you are going to build some kernel or driver for any type of hardware(which is in the list of QEmu) without having the hardware you can build your task.
Scratchbox2 is cross-compilation toolkit designed to make embedded Linux application development easier. It also provides a full set of tools to integrate and cross-compile an entire Linux distribution. It is also having the QEmu and other tools required to build a particular kernel or driver for the hardware.