libvirt: Add test module for virsh {at|de}tach-disk

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt: Modify virsh function for virsh {at|de}tach-disk

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt: Add get_disk_count() for virsh {at|de}tach-disk

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #304 from liyangfnst/attach_detach_disk

Add test for virsh {at|de}tach-disk