Iscsidev.setup: Set selinux to permissive mode before login

If selinux is enforcing, when adding a logicalunit to a target during
iscsi target epxport, you will get the error: 'tgtadm: invalid request'.
So compromise selinux mode to avoid this problem.

Signed-off-by: Yanbing Du <ydu@redhat.com>
We are using two different modules here to implement what we want. But I think we can add a function in utils_selinux named is_enforcing() and is_permissive(). Then we do not have to depend on the utils_misc when we want to get status of selinux. 

Considering the point mentioned above, I sent a PR to add a is_enforcing() in utils_selinux.https://github.com/autotest/virt-test/pull/1696
Then you can use it here I think. Thanx.