nvme_sim: signal namespace depature

Signal when the namespace is gone so we can tear down the disk when a
nvme drive is removed.

Approved by: re (cperciva)
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D33032

(cherry picked from commit 4640f5008922c5b189d2f7b63edf73300277e6df)
(cherry picked from commit b7ffac04541f98d3d8c6078092d59f0fed88b1ef)