dev-storage: Fix the unusual function name

The function name of usb_msd_{realize,unrealize}_*,
usb_msd_class_initfn_* are unusual. Rename it to
usb_msd_*_{realize,unrealize}, usb_msd_class_*_initfn.

Cc: Gerd Hoffmann <kraxel@redhat.com>

Signed-off-by: Mao Zhongyi <maozy.fnst@cn.fujitsu.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 11e6003433abce35f3f4970e1acc71ee92dbcf51.1511317952.git.maozy.fnst@cn.fujitsu.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
