Default to not performing the early-boot memory tests when we detect we

Default to not performing the early-boot memory tests when we detect we
are booting inside a VM.  There are three reasons to disable this:

o  It causes the VM host to believe that all the tested pages or RAM are
   in use.  This in turn may force the host to page out pages of RAM
   belonging to other VMs, or otherwise cause problems with fair resource
   sharing on the VM cluster.
o  It adds significant time to the boot process (around 1 second/Gig in
   testing)
o  It is unnecessary - the host should have already verified that the
   memory is functional etc.

Note that this simply changes the default when in a VM - it can still be
overridden using the hw.memtest.tests tunable.

MFC after:	4 weeks