virt: Fix a bug in Iscsidev init func

Set 'self.emulated_image' to be a true file, if not
cleanup cannot find the emulated_image file.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #1734 from liyangfnst/iscsi_fix

virt: Fix a bug in Iscsidev init func