block/iscsi: do not forget to logout from target

We actually were always impolitely dropping the connection and
not cleanly logging out.

CC: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Message-id: 1429193313-4263-2-git-send-email-pl@kamp.de
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 20474e9a)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>