bhyve nvme: Inform guests of namespace resize

Register a "block resize" callback to be notified of changes to the
backing storage for the Namespace. Use this to generate an Asynchronous
Event Notification, Namespace Attributes Changed when the guest OS
provides an Asynchronous Event Request.

(cherry picked from commit cf3ed8e0cd9505f21f614e8208f7afd9cfa4d621)