xend/pvscsi: fix passing of SCSI control LUNs

Currently pvscsi can not pass SCSI devices that have just a scsi_generic node.
In the following example sg3 is a control LUN for the disk sdd.
But vscsi=['4:0:2:0,0:0:0:0'] does not work because the internal 'devname'
variable remains None. Later writing p-devname to xenstore fails because None
is not a valid string variable.

Since devname is used for just informational purpose use sg also as devname.

carron:~ $ lsscsi -g
[0:0:0:0]    disk    ATA      FK0032CAAZP      HPF2  /dev/sda   /dev/sg0
[4:0:0:0]    disk    HP       P2000G3 FC/iSCSI T100  /dev/sdb   /dev/sg1
[4:0:1:0]    disk    HP       P2000G3 FC/iSCSI T100  /dev/sdc   /dev/sg2
[4:0:2:0]    storage HP       HSV400           0950  -         /dev/sg3
[4:0:2:1]    disk    HP       HSV400           0950  /dev/sdd   /dev/sg4
[4:0:3:0]    storage HP       HSV400           0950  -         /dev/sg5
[4:0:3:1]    disk    HP       HSV400           0950  /dev/sde   /dev/sg6

---
 tools/python/xen/util/vscsi_util.py |    2 ++
 1 file changed, 2 insertions(+)

Index: xen-4.1.2-testing/tools/python/xen/util/vscsi_util.py
===================================================================
--- xen-4.1.2-testing.orig/tools/python/xen/util/vscsi_util.py
+++ xen-4.1.2-testing/tools/python/xen/util/vscsi_util.py
@@ -105,6 +105,8 @@ def _vscsi_get_scsidevices_by_lsscsi(opt
             devname = None
         try:
             sg = s[-1].split('/dev/')[1]
+            if devname is None:
+                devname = sg
             scsi_id = _vscsi_get_scsiid(sg)
         except IndexError:
             sg = None