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>
So what's the problem, I didn't seen any fail in my test....
```
00:06:43 DEBUG| Running virsh command: attach-disk --domain vm1 --source /home/autotest_fjtest_v1.1/client/tests/virt/shared/attach.img --target vdb --driver qemu --mode readonly --shareable
00:06:43 DEBUG| Running '/usr/bin/virsh attach-disk --domain vm1 --source /home/autotest_fjtest_v1.1/client/tests/virt/shared/attach.img --target vdb --driver qemu --mode readonly --shareable'
00:06:43 DEBUG| status: 0
00:06:43 DEBUG| stdout: Disk attached successfully
00:06:43 DEBUG| stderr:
```
Do you have some error message for this problem?