bhyve: add QEMU_FWCFG_INDEX_MAX_CPUS item

Requested-by: coreboot
Reviewed by: <If someone else reviewed your modification.>
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39315

(cherry picked from commit 305edaa47918a559de46a5d72060644009f1875c)