Avoid said the /dev/xvd* is already defined after

open a vhd file failed

For example:
[root@dom0_134_103 ~]# xm block-attach 0
tap2:tapdisk:vhd:/root/liyi***.vhd /dev/xvdz  w
Error: ('create', '-avhd:/root/liyi***.vhd') failed (512  )
Usage: xm block-attach <Domain> <BackDev> <FrontDev> <Mode> [BackDomain]
Create a new virtual block device.

[root@dom0_134_103 ~]# xm block-attach 0
tap2:tapdisk:vhd:/root/liyi.vhd /dev/xvdz  w
Error: The device "/dev/xvdz" is already defined
Usage: xm block-attach <Domain> <BackDev> <FrontDev> <Mode> [BackDomain]

Create a new virtual block device.

Signed-off-by: Yi Li <peteryili@tencent.com>
Acked-by: Matt Wilson <msw@amazon.com>

tools/python/xen/xend/XendDomainInfo.py

@@ -65,7 +65,7 @@ from xen.xend.XendConstants import *
 from xen.xend.XendAPIConstants import *
 from xen.xend.XendCPUPool import XendCPUPool
 from xen.xend.server.DevConstants import xenbusState
-from xen.xend.server.BlktapController import TapdiskController
+from xen.xend.server.BlktapController import TapdiskController, TapdiskException
 
 from xen.xend.XendVMMetrics import XendVMMetrics
 
@@ -861,7 +861,7 @@ class XendDomainInfo:
                     # blktap1
                     dev_type = self.getBlockDeviceClass(devid)
                 self._waitForDevice(dev_type, devid)
-            except VmError, ex:
+            except (VmError, TapdiskException), ex:
                 del self.info['devices'][dev_uuid]
                 if dev_type == 'pci':
                     for dev in dev_config_dict['devs']: