bhyve: Don't force an upper bound on vCPUs when parsing pinning.

Even today it is possible to specify pinning for a vCPU higher than
the configured number of CPUs but lower than VM_MAXCPU without raising
an error.

Reviewed by: grehan
Differential Revision: https://reviews.freebsd.org/D34492

(cherry picked from commit fd6f92946f02b451c792bfdbc94259ff65e49969)