engines/xnvme: add xnvme engine

engines/xnvme: add xnvme engine

This patch introduces a new fio engine to work with xNVMe >= 0.2.0.
xNVMe provides a user space library (libxnvme) to work with NVMe
devices. The NVMe driver being used by libxnvme is re-targetable and
can be any one of the GNU/Linux Kernel NVMe driver via libaio,
IOCTLs, io_uring, the SPDK NVMe driver, or your own custom NVMe driver.

For more info visit https://xnvme.io
https://github.com/OpenMPDK/xNVMe

Co-Authored-By: Ankit Kumar <ankit.kumar@samsung.com>
Co-Authored-By: Simon A. F. Lund <simon.lund@samsung.com>
Co-Authored-By: Mads Ynddal <m.ynddal@samsung.com>
Co-Authored-By: Michael Bang <mi.bang@samsung.com>
Co-Authored-By: Karl Bonde Torp <k.torp@samsung.com>
Co-Authored-By: Gurmeet Singh <gur.singh@samsung.com>
Co-Authored-By: Pierre Labat <plabat@micron.com>
Link: https://lore.kernel.org/r/20220511163019.5608-2-ankit.kumar@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>