Fix NVMe's use of XPT_GDEV_TYPE

This patch changes the way XPT_GDEV_TYPE works for NVMe. The current
ccb_getdev structure includes pointers to the NVMe Identify Controller
and Namespace structures, but these are kernel virtual addresses which
are not accessible from user space.

As an alternative, the patch changes the pointers into padding in
ccb_getdev and adds two new types to ccb_dev_advinfo to retrieve the
Identify Controller (CDAI_TYPE_NVME_CNTRL) and Namespace
(CDAI_TYPE_NVME_NS) data structures.

Reviewed By: rpokala, imp
Differential Revision: https://reviews.freebsd.org/D10466
Submitted by: Chuck Tuffli