interface: introduce downscript element for interface

interface: introduce downscript element for interface

https://gitlab.com/libvirt/libvirt/-/issues/13

Add support for downscript:

<interface type='ethernet'>
    <mac address='00:11:22:33:44:55'/>
    <script path='/etc/qemu-ifup'/>
    <downscript path='/path/to/my/downscript'/>
</interface>

Signed-off-by: Chen Hanxiao <chen_han_xiao@126.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>