nvme: Add support for Read Data and Write Data in CMBs.

nvme: Add support for Read Data and Write Data in CMBs.

Add the ability for the NVMe model to support both the RDS and WDS
modes in the Controller Memory Buffer.

Although not currently supported in the upstreamed Linux kernel a fork
with support exists [1] and user-space test programs that build on
this also exist [2].

Useful for testing CMB functionality in preperation for real CMB
enabled NVMe devices (coming soon).

[1] https://github.com/sbates130272/linux-p2pmem
[2] https://github.com/sbates130272/p2pmem-test

Signed-off-by: Stephen Bates <sbates@raithlin.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>