bhyve: NVMe set nominal health values

Some operating systems believe bhyve's emulated NVMe drive is failing
based on certain values in the SMART / Health Information log page being
zero. Fix is to set the reported temperature and available spare values
to reasonable defaults.

Submitted by: wanpengqian@gmail.com
Reviewed by: grehan
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24202