Multiple NVMe controllers within a subsystem does not work correctly,
since we would need to virtualize the controller data, namespace IDs,
and so on. For now, only allow pass-through mapping of a single NVMe
controller per subsystem.
Change-Id: Ib2d3576d2856c46a086f38eb6bec56f3e7a73575
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>