libvirt.test: Add more options test for attach-disk

Add some more test for attach-disk, for example:
--serial, --address, --multifunction, --subdriver, etc.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Hmmm, that's a problem when we need to attach disk with mode are both readonly and shareable. I think it is a bug in libvirt. BTW, in my last comment, it is not a link, it is a commit id in libvirt. But now I think the problem is that:

Your patch here make one test case lost quietly. 
```
virsh attach-disk ... --mode readonly
```
It means, after your patch here, tp-libvirt does not test attach-disk with --mode readonly any more. So, from this viewpoint, I believe it is a regression for tp-libvirt. So I send a PR to add this test case back. When we extend the test in tp-libvirt, at least we should keep it cover the all test cases we already have in it, right?

About how to attach-disk both with readonly and shareable, yes, we can add another case to test it. And I believe there is a problem in upstream of libvirt and I have sent a patch to libvir-list for it. At least we can verify that it is a problem from the libvirt guys. 

Before we are sure that the usage of attach disk with both modes, we should remove this test case from our tp-libvirt to void noise to tester, agree?

If so, please go to take a look at my PR #202 . If it is okey to you, please merge it as soon. Then we can make this problem not confuce another user as possible. 

When we make sure the usage how to attach disk with readonly and shareable, we can add a case to cover it. Even though, we should not remove the case only for readonly.