virttest: Init export_flag in __init__ function of iscsi

As export_flag will be used in cleanup() so init it no matter we are
using emulate or real iscsi device.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
virttest: Import iscsi from virttest first

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
virttest: Update iscsi_unittest to fit the update in iscsi

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1619 from ypu/iscsi-small-fix

Iscsi small fix