iscsi.export_target: change selinux mode

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>
iscsi: Support CHAP authentication for iscsi target

1. Implement CHAP authentication setup.
2. Update libvirt secret_xml to use CHAP authentication.

Signed-off-by: Yanbing Du <ydu@redhat.com>
libvirt_xml.devices.disk: Support auth element for disk xml

Signed-off-by: Yanbing Du <ydu@redhat.com>
utils_test.libvirt: Support more disk type for create_disk_xml

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #1695 from will-Do/iscsi_selinux

iscsi: Support CHAP authentication for iscsi target and other related updates