core: Disallow RO LUN ISCSI disks

Qemu currently does not support RO Direct-LUN disks connected using
Virt-IO-SCSI.
This patch adds a validation to block this in AddDiskCommand CDA.
Also added a new VdcBllMessage for this case.

Change-Id: Id84346098d42f20c593c2682dc9ff99a3e77d534
Bug-Url: https://bugzilla.redhat.com/1082673
Signed-off-by: Vered Volansky <vvolansk@redhat.com>