For each queue doorbell registers are paired as:
- Submission Queue Tail Doorbell
- Completion Queue Head Doorbell

Reflect that in the NVMeRegs structure, and adapt
nvme_create_queue_pair() accordingly.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200904124130.583838-4-philmd@redhat.com>
Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Fam Zheng <fam@euphon.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>