bhyve: Fix Number of Power States Supported value

Summary:
Set Number of Power States Supported to indicate 1 power state. Keep the
Power State Descriptor data structures as zero to indicate "Not
reported".

Test Plan:
Within FreeBSD/Linux guests, list the number of power states and check
the Max Power value.

(cherry picked from commit eae0210cdd2e22c2eb9bc652f2136b3386a663a4)