See this thread in the Virtualization Forum.
Should improve bootup times for any Guest using any of the major virtualization technologies I've tested on so far (Of course not Linux Containers of any type).

In the forum thread I also posted a link to a small script I created, you only need to follow the instructions to run the script once to implement the fix.

This is a longstanding hardly noticed issue until now. Looking back at my old virtual machines, this goes at least as far back as openSUSE 12.1, maybe further.