vkernel - Change default pre-zeroing of main memory

vkernel - Change default pre-zeroing of main memory

* Do not pre-zero memory by default, because it interferes with the
  ability to feed NUMA associations down into the vkernel.

* Add the -z option to force pre-zeroing of memory, which can be
  useful when doing basic benchmarking (to avoid the overhead of the
  vkernel having to do initial-pageins of its main memory, which can
  extend over a long period of time).