bhyve nvme: Support minimal Controller list

Controllers must support the Identify Controller list if they support
Namespace Management. But the UNH NVMe tests use this command regardless
of whether the device under test supports Namespace Management.

This implementation returns an empty Controller list (i.e., Number of
Identifiers is zero).

Fixes UNH Test 1.1.2

(cherry picked from commit 715f82e4f5c51c04314904dee8090c8abbbe2156)