block_iscsi_lvm: Assign unattended delivery method explicitly

Assign unattended delivery method to cdrom explicitly in the
scenarios where unatteneded install guest OS form extra cdrom.
Signed-off-by: NYongxue Hong <yhong@redhat.com>

qemu/tests/cfg/block_iscsi_lvm.cfg

@@ -76,6 +76,7 @@
             variants:
                 - extra_cdrom_ks:
                     no WinXP Win2000 Win2003 WinVista
+                    unattended_delivery_method = cdrom
                     cdroms += " unattended"
                     drive_index_unattended = 3
                     drive_index_cd1 = 1