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

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>