virt-libvirt: add configuration for virsh dommemstat

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

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

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