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>
Merge pull request #140 from liyangfnst/attach_disk

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