virtual_network/ethernet: select interface more reliably (#6556)

Previously, the code used `interface_base.get_vm_iface` to select what
ever that function determined to be the first guest interface inside the
guest. However, this selection could fail leading to a mismatch in MTU
values in test cases that have two interfaces on the VM.

Instead, use `utils_net_get_linux_iface_info` to select the interface
more reliably via the mac that's given by the previously obtained
interface information from the VM XML. This assures that the vm
interface corresponds to the selected domain xml element.

Assisted-by: Cursor/Claude-4-Sonnet

Signed-off-by: Sebastian Mitterle <[email protected]>

Author: smitterl

libvirt/tests/src/virtual_network/connectivity/connectivity_check_ethernet_interface.py

@@ -12,7 +12,6 @@
 from virttest.utils_libvirt import libvirt_vmxml
 from virttest.utils_test import libvirt
 
-from provider.interface import interface_base
 from provider.virtual_network import network_base
 
 LOG = logging.getLogger('avocado.' + __name__)
@@ -175,9 +174,10 @@ def run(test, params, env):
                 LOG.info('MTU check of vmxml PASS')
 
             # Check mtu inside vm
-            vm_iface = interface_base.get_vm_iface(session)
-            vm_iface_info = utils_net.get_linux_iface_info(iface=vm_iface,
+            iface_mac = iface['mac_address']
+            vm_iface_info = utils_net.get_linux_iface_info(mac=iface_mac,
                                                            session=session)
+            vm_iface = vm_iface_info['ifname']
             vm_mtu = vm_iface_info['mtu']
             if int(vm_mtu) != int(iface_mtu):
                 test.fail(f'MTU of interface inside vm should be '