migration: new case updating interface during migration (#6611)

This commit adds test cases for updating network interface MTU settings
in guest XML configuration during live migration scenarios.

The motivation is to ensure network configuration integrity during
complex migration procedures, preventing connectivity issues that may
arise from inconsistent MTU settings. The approach involves modifying
MTU values at different migration stages and validating network
functionality post-migration.

Reference: xxxx-298951

Signed-off-by: nanli <[email protected]>

Author: nanli1

libvirt/tests/cfg/migration/migration_xml_update.cfg

@@ -45,4 +45,13 @@
                     new_address = <address type='pci' domain='0x0000' bus='0x00' slot='0x1B' function='0x7'/>
                     s390-virtio:
                         new_address = <address type='ccw' cssid='0xfe' ssid='0x0' devno='0x9999'/>
+                - update_interface_mtu:
+                    remove_all = interface
+                    modify = mtu
+                    status_error = yes
+                    original_mtu = "5000"
+                    updated_mtu = "6000"
+                    add_device = {"type_name": "network", "source": {"network": "default"}, "model": "virtio", "mtu": {"size": "${original_mtu}"}}
+                    err_msg = "unsupported configuration: Target network card MTU .* does not match source .*"
+
 

libvirt/tests/src/migration/migration_xml_update.py

@@ -55,6 +55,11 @@ def create_migration_xml(test, vm_name, params):
         old_address = dev.find("address")
         dev.remove(old_address)
         dev.append(new_address)
+    elif modify == "mtu":
+        # Update MTU size for interface element
+        mtu_element = dev.find("mtu")
+        mtu_element.attrib['size'] = params.get("updated_mtu")
+
     ElementTree.ElementTree(vmxml).write(tmp_file)