Fix block-detach of non-existent device

diff -r ec84b119e4ed tools/python/xen/xend/server/DevController.py
@@ -78,8 +78,12 @@
         backpath = xstransact.Read(frontpath, "backend")
 
         xstransact.Remove(frontpath)
-        xstransact.Remove(backpath)
-
+
+        if backpath:
+            xstransact.Remove(backpath)
+        else:
+            raise VmError("Device not connected")
+           
 
     def configurations(self):
         return map(lambda x: self.configuration(int(x)),

diff -r ec84b119e4ed tools/python/xen/xm/main.py
@@ -165,6 +165,9 @@
     error = str(ex)
     if error == "Not found" and dom != None:
         err("Domain '%s' not found when running 'xm %s'" % (dom, cmd))
+        sys.exit(1)
+    elif error == "Exception: Device not connected":
+        err("Device not connected")
         sys.exit(1)
     else:
         raise ex

@@ -532,7 +535,12 @@
     arg_check(args,2,"block-detach")
 
     dom = args[0]
-    dev = args[1]
+
+    try:
+        dev = int(args[1])
+    except ValueError, e:
+        err("Invalid device id: %s" % args[1])
+        sys.exit(1)
 
     from xen.xend.XendClient import server
     server.xend_domain_device_destroy(dom, 'vbd', dev)