bhyve: abort and return FEATURE_NOT_SAVEABLE while set feature with a save flag for NVMe controller.

Currently bhyve's NVMe controller cannot save feature values cross
reboot. It should return a FEATURE_NOT_SAVEABLE error when the command
specifies a save flag.

Quote from NVMe specification, page 205:

https://nvmexpress.org/wp-content/uploads/NVM-Express-1_4-2019.06.10-Ratified.pdf

If the Feature Identifier specified in the Set Features command is not
saveable by the controller and the controller receives a Set Features
command with the Save bit set to one, then the command shall be aborted
with a status of Feature Identifier Not Saveable.

Reviewed by: chuck (older version)
Approved by: manu (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D32767