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>
OK, I got your mean. Seems virt-manager has this problem too, :( Because I can set the disk to be both modes manually by virt-manager(although it's not in our test field).