nvmet: add error log support for fabrics-cmd

This patch adds the support to maintain error log page for the fabrics
prop get, prop set, and admin connect commands. Here we also update the
discovery.c and add update set/get features and parse functions to
support error log page.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>