bhyve: add varfile option to nvlist of lpc device

Use seperate nvlist entries for the romfile and the varfile.

While here, don't leak varfd in bootrom_loadrom().

Reviewed by: jhb, markj
Differential Revision: https://reviews.freebsd.org/D33433

(cherry picked from commit 87f6367f10614f58e5f93130b7be3364d2f83068)