update svirt_attach_detach_disk test script.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
fix sync method to restore if define() fail.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Update the VM attach_disk and detach_disk method.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>