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

(cherry picked from commit 05a21658eb15463af9427fc1f7825b2aa244b9d5)