multipath.py: add Ubuntu scsi_id path when searching for scsi_id

The patch is on going work of porting VDSM to Ubuntu
http://www.ovirt.org/VDSM_on_Ubuntu

Change-Id: I97aa86946782180bad3926c50ac71a6eb60de138
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/13086
Reviewed-by: Dan Kenigsberg <danken@redhat.com>