virt-libvirt: add configuration for virsh vncdisplay

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

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

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