bhyve: Remove vestigial support for setting max vCPUs.

The kernel part of the hypervisor is not going to support per-VM maxcpu
limits. The topology is only used to control the values returned by
CPUID leaves for which max vCPUs is not relevant.

Reviewed by: corvink, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37176