Export cinder volumes only if the status is 'in-use'

Currently, cinder volumes are exported both 'in-use' and 'available'
after restarting cinder-volume service.
This behavior was introduced following commit.

  commit ffefe18334a9456250e1b6ff88b7b47fb366f374
  Author: Zhiteng Huang <zhithuang@ebaysf.com>
  Date: Sat Aug 23 18:32:57 2014 +0000

If the volumes are attached to nova instances, they should be exported
via tgtd after restarting cinder-volume.
But the volumes which are not attached to instances must not be exported
because everyone can connect these volumes.

This patch changes volume export behavior that exports a volume only if
the volume status is 'in-use'.

Change-Id: I4c598c240b9290c81bd8001e5a0720c8c329aeb9
Signed-off-by: Mitsuhiro Tanino <mitsuhiro.tanino@hds.com>
Closes-bug: #1381106
(cherry picked from commit e2f28b967910625432be0eab6a851adf53ac58ea)