hw/block/nvme: Convert to realize

Convert nvme_init() to realize and rename it to nvme_realize().

Cc: John Snow <jsnow@redhat.com>
Cc: Keith Busch <keith.busch@intel.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>

Signed-off-by: Mao Zhongyi <maozy.fnst@cn.fujitsu.com>
Message-id: 2882e72d795e04cbe2120f569d551aef2467ac60.1511317952.git.maozy.fnst@cn.fujitsu.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>