rename nvme_ctrlr_destroy_qpair to nvme_ctrlr_destroy_qpairs

Maintain symmetry with nvme_ctrlr_create_qpairs, making it easier to
match init/uninit scenarios.

Signed-off-by: John Meneghini <johnm@netapp.com>
Submitted by: Michael Hordijk <hordijk@netapp.com>
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D19781