libvirt.test: Fix a bug for attach-device

In old libvirt version, '--config' and '--persistent'
cannot be used together. and we'd better test them
separately.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #270 from liyangfnst/attach_device_fix

libvirt.test: Fix a bug for attach-device