libvirt: Add test module for virsh domblkinfo.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt: Add virsh function for virsh domblkinfo.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt: Add get_disk_source() for virsh domblkinfo.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>