nvmf/vfio-user: drop struct nvme_q

There is very little now shared between submission and completion
queues, so drop usage of this struct, folding its remaining members
into the relevant owning types.

Signed-off-by: John Levon <john.levon@nutanix.com>
Change-Id: I02195d1944ca9905ef03ddf2c099ddb806df70dc
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/11296


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Community-CI: Mellanox Build Bot
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>