qemu_capabilities: Fill supported net backend types

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

tests/domaincapsdata/qemu_8.2.0.armv7l.xml

@@ -161,6 +161,12 @@
         <value>lkcf</value>
       </enum>
     </crypto>
+    <interface supported='yes'>
+      <enum name='backendType'>
+        <value>default</value>
+        <value>passt</value>
+      </enum>
+    </interface>
   </devices>
   <features>
     <gic supported='no'/>