nvmf/vfio-user: eliminate `nvmf_vfio_user_qpair`

Previously we mixed SQ/CQ definition together, one queue pair
data structure may contain CQ,SQ or both CQ and SQ separately,
while here, we split the queue pair definition into SQ and CQ
respectively as code cleanup.

The NVMf library uses queue pair concept, but for vfio-user
case, each SQ created by VM is mapped to NVMf queue pair, so
we also change `connected_qps` to `connected_sqs` to reflect
the fact.

No actual code logic change in this commit.

Change-Id: I293ccbfbf054fe864d348fc56793dd1ccd366f6d
Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/11036


Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>