iscsi: add HE disk to the engine as direct lun

Add Hosted Engine disk to the engine as direct lun in order to allow
the engine to know about the LUN used by Hosted engine, so the
engine can block actions like:

- import the hosted-engine's storage domain
- put the iSCSI domain in maintenance while the hosted-engine is installed on it
- pick the lun for storage domain creation/extension

Change-Id: I9c5640bde713fc4d09cf74a7037c62e833495934
Bug-Url: https://bugzilla.redhat.com/1157243
Bug-Url: https://bugzilla.redhat.com/1157239
Bug-Url: https://bugzilla.redhat.com/1157238
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>