Raise an exception if blkdev_name_to_number fails, to improve the error message.

Fix came from Glauber de Oliveira Costa, and is one half of xen-unstable
11831:f5321161c649fe4f07027250c4a1f3c4b5fc2ed9.

Signed-off-by: Ewan Mellor <ewan@xensource.com>

tools/python/xen/xend/server/blkif.py

@@ -80,6 +80,9 @@ class BlkifController(DevController):
                          'acm_policy' : policy})
 
         devid = blkif.blkdev_name_to_number(dev)
+        if not devid:
+            raise VmError('Unable to find number for device (%s)' % (dev))
+
         front = { 'virtual-device' : "%i" % devid,
                   'device-type' : dev_type
                 }