webadmin: attach disk - allow read-only for passthrough

Attach disk dialog:
allow read-only selection for DirecLUN disks using
SCSI pass-through.

Change-Id: Id2ff8730b7da1d20e71339871ee462c23d263c5b
Bug-Url: https://bugzilla.redhat.com/1118847
Signed-off-by: Daniel Erez <derez@redhat.com>