bhyve: return FEATURE_NOT_CHANGEABLE for unimplemented feature of NVMe controller

Set Feature is a feature specified function. Currently only some
features have the set procedure. For features that are not handled by
the controller, we should return a FEATURE_NOT_CHANGEABLE error message.

Approved by: manu (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D32802