Fix a bug in Iscsi.export_target func

If emulated_image already exists, but size is different with
you want, we must rebuild this file, if not, test will fail.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #1735 from liyangfnst/emulate

Fix a bug in Iscsi.export_target func