Query MVPConf0.PVPE for number of CPUs.

Rather than hard-coding the number of CPUs to 2, look up the PVPE field
in MVPConf0, as the valid VPE numbers are from 0 to PVPE inclusive.

Submitted by: "James Clarke" <jrtc4@cam.ac.uk>
Reviewed by: br
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D16644