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

Reviewed by: jhb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D36193