virsh_attach_detach_disk: fix rhel version check.

As we need to run tests not only for rhel guest, we can not
assume "rpm -qa|grep redhat-release" should return 0.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
virsh_attach_detach_disk: add test case for --persistent option.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Merge pull request #104 from yangdongsheng/at-detach-disk

virsh-tests: Add --persistent for at-detach-disk test