bhyve: Support a _VARS.fd file for bootrom

OVMF creates two separate .fd files, a _CODE.fd file containing
the UEFI code, and a _VARS.fd file containing a template of an
empty UEFI variable store.

OVMF decides to write variables to the memory range just below the
boot rom code if it detects a CFI flash device. So here we add
just the barest facsimile of CFI command handling to bootrom.c
that is needed to placate OVMF.

Submitted by: D Scott Phillips <d.scott.phillips@intel.com>
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D19976
MFC After: 1 week