This patch marks QED images as clean periodically when it is safe to do so. This reduces the chance of having to perform a consistency check at startup. Previously we left the image dirty even when it was consistent, therefore risking an unnecessary consistency check after crash.
The first patch stubs out qemu-timer.c functions for qemu-tool.c, which is needed since block drivers get built into qemu-img and friends. In the long run having timers in qemu-tool too would be good, but for now they are not critical and the stub implementation works.