An error occurs if the first detach was successful and the 2nd one is

not (of course). Return if the 1st detach is successful, try the 2nd one
if it was not successful. Throw an error if the 2nd one fails.
This should also fix problems I am currently seeing in the xm-test
suite.

Signed-off-by: Stefan Berger <stefanb@us.ibm.com>

tools/python/xen/xm/main.py

@@ -1556,7 +1556,11 @@ def detach(args, command, deviceClass):
 
 
 def xm_block_detach(args):
-    detach(args, 'block-detach', 'vbd')
+    try:
+        detach(args, 'block-detach', 'vbd')
+        return
+    except:
+        pass
     detach(args, 'block-detach', 'tap')