libnvme-mi: Introduce NVMe Managament Interface library

libnvme-mi: Introduce NVMe Managament Interface library

This change adds support for the NVMe-MI protocol, as a secondary
library: libnvme-mi.{a,so,h}. This allows management of NVMe devices,
typically through NVMe Admin Channel commands, transmitted over a
side-band channel - typically MCTP over SMBus.

There's a fair amount of shared structure between the direct-ioctl and
MI channel implementations; for example, the Admin command set is used
mostly as-is over the MI transport. The library is built as separate
.so/.a objects as client applications would typically either use one or
the other. MI-specific functions are added with a 'nvme_mi' prefix.

This change introduces a small set of MI commands using this channel.
We'll extend this to further MI commands in subsequent changes, as well
as implement Admin Channel commands too.

We currently assume a MCTP transport for NVMe-commands, using the new
AF_MCTP socket support in Linux. However, the transport-specific code is
kept somewhat separate, through the internal struct nvme_mi_transport
interface. This allows potential alternative transports in future - for
example in-band PCIe.

Finally, we start a new example application, mi-mctp, which provides
a reference application using the new library.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>