bhyve nvme: Advertise Namespace changed AEN

Advertise Namespace Attribute Notices events in the Optional
Asynchronous Events Supported (OAES) field of the Identify Controller
data structure. Additionally, rename the enums and macros to clarify
these are AEN's related to Notices and not generic information.

Reported by: andy@omniosce.org

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D34331