Tested on a desktop-class system with 16GB of memory, creating
16GB raw bhyve images. With this change, I see an order of magnitude
less swap usage after creating an image. I also see a large runtime
improvement if most of the system's dirty anonymous memory has
not already been swapped out: 50s down to 20s. Once most inactive
pages have been swapped out, there's no significant difference in
runtime.