nvme: Add support for Controller Memory Buffers

Implement NVMe Controller Memory Buffers (CMBs) which were added in
version 1.2 of the NVMe Specification. This patch adds an optional
argument (cmb_size_mb) which indicates the size of the CMB (in
MB). Currently only the Submission Queue Support (SQS) is enabled
which aligns with the current Linux driver for NVMe.

Signed-off-by: Stephen Bates <sbates@raithlin.com>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>