virt-libvirt: add get_disk_all() and get_disk_blk() function for virsh domblkstat

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

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt-libvirt: add test for virsh domblkstat

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