fix(interfaces): deleting a bridge keeps the devices it held

With the netcf backend (RHEL/AlmaLinux 8), undefining a bridge also
removes its members' configuration: the device came back without any
after a reboot. Like virsh iface-unbridge, each member is now defined
again on its own after the bridge is undefined, with the bridge's start
mode and, when it is the only member, its addresses. A bond keeps its
mode and devices.

Checked on AlmaLinux 8 (libvirt 8.0, netcf): an ethernet and a bond
member get their ifcfg back, and the bond comes up with the address
after a reboot.

interfaces/tests/test_delete.py

@@ -0,0 +1,44 @@
+"""Deleting a host interface: netcf removes a bridge's members with the
+bridge, so they are defined again on their own (as virsh iface-unbridge)."""
+
+from unittest.mock import MagicMock
+
+from django.test import SimpleTestCase
+from lxml import etree
+from vrtManager.interface import wvmInterface
+
+ADDRESS = "<protocol family='ipv4'><ip address='192.0.2.10' prefix='24'/></protocol>"
+BOND = "<interface type='bond' name='bond0'><bond mode='active-backup'><interface type='ethernet' name='eth1'/></bond></interface>"
+
+
+def delete(xml):
+    conn = wvmInterface.__new__(wvmInterface)
+    conn.iface = MagicMock()
+    conn.iface.XMLDesc.return_value = xml
+    conn.wvm = MagicMock()
+    conn.delete_iface()
+    conn.iface.undefine.assert_called_once_with()
+    return [etree.fromstring(call.args[0]) for call in conn.wvm.interfaceDefineXML.call_args_list]
+
+
+class DeleteInterfaceTestCase(SimpleTestCase):
+    def test_the_only_member_takes_the_bridge_start_mode_and_addresses(self):
+        (bond,) = delete(f"<interface type='bridge' name='br1'><start mode='onboot'/>{ADDRESS}<bridge>{BOND}</bridge></interface>")
+        self.assertEqual(bond.get("name"), "bond0")
+        self.assertEqual(bond.xpath("bond/@mode"), ["active-backup"])
+        self.assertEqual(bond.xpath("bond/interface/@name"), ["eth1"])
+        self.assertEqual(bond.xpath("start/@mode"), ["onboot"])
+        self.assertEqual(bond.xpath("protocol/ip/@address"), ["192.0.2.10"])
+
+    def test_several_members_take_the_start_mode_but_no_address(self):
+        members = delete(
+            f"<interface type='bridge' name='br1'><start mode='onboot'/>{ADDRESS}<bridge>"
+            "<interface type='ethernet' name='eth1'/><interface type='ethernet' name='eth2'/></bridge></interface>"
+        )
+        self.assertEqual([m.get("name") for m in members], ["eth1", "eth2"])
+        for member in members:
+            self.assertEqual(member.xpath("start/@mode"), ["onboot"])
+            self.assertEqual(member.xpath("protocol"), [])
+
+    def test_an_interface_that_is_not_a_bridge_is_only_undefined(self):
+        self.assertEqual(delete(f"<interface type='ethernet' name='eth1'><start mode='onboot'/>{ADDRESS}</interface>"), [])