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>

docs/formatdomain.html.in

@@ -5879,8 +5879,13 @@
     <p>
       After creating/opening the tap device, an optional shell script
       (given in the <code>path</code> attribute of
-      the <code>&lt;script&gt;</code> element) will be run; this can
-      be used to do whatever extra host network integration is
+      the <code>&lt;script&gt;</code> element) will be run.
+      <span class="since">Since 0.2.1</span>
+      Also, after detaching/closing the tap device, an optional shell
+      script (given in the <code>path</code> attribute of
+      the <code>&lt;downscript&gt;</code> element) will be run.
+      <span class="since">Since 5.1.0</span>
+      These can be used to do whatever extra host network integration is
       required.
     </p>
 
@@ -5889,6 +5894,7 @@
 &lt;devices&gt;
   &lt;interface type='ethernet'&gt;
     &lt;script path='/etc/qemu-ifup-mynet'/&gt;
+    &lt;downscript path='/etc/qemu-ifdown-mynet'/&gt;
   &lt;/interface&gt;
   ...
   &lt;interface type='ethernet'&gt;

docs/schemas/domaincommon.rng

@@ -3191,6 +3191,14 @@
           <empty/>
         </element>
       </optional>
+      <optional>
+        <element name="downscript">
+          <attribute name="path">
+            <ref name="filePath"/>
+          </attribute>
+          <empty/>
+        </element>
+      </optional>
       <optional>
         <element name="backenddomain">
           <attribute name="name">