test: Avoid virsh iface-list

This depends on the deprecated `virtinterfaced` which we are getting rid
of. This code was both unnecessarily complicated, and also too
arbitrary: It just took what interface appeared in `virsh iface-list`
first.

Change this to be more realistic: The test creates a new network which
NATs to a host device, so in most practical cases that should be the one
having the default route. Rename the function and add a comment to
clarify this.

test/check-machines-networks

@@ -17,24 +17,17 @@
 # You should have received a copy of the GNU Lesser General Public License
 # along with Cockpit; If not, see <http://www.gnu.org/licenses/>.
 
+import json
 import xml.etree.ElementTree as ET
 
 import machineslib
 import testlib
 from machinesxmls import TEST_NETWORK2_XML, TEST_NETWORK3_XML, TEST_NETWORK4_XML, TEST_NETWORK_IPV6_XML, TEST_NETWORK_XML
 
 
-def getNetworkDevice(m):
-    net_devices_str = m.execute("virsh iface-list")
-    net_devices_str = net_devices_str.split("\n", 2)[2]  # Remove first 2 lines of table header
-    if net_devices_str not in ["\n", "\r\n"]:
-        device = net_devices_str.split(' ', 2)[1]  # Get the name of device, ignoring spacing before device string
-    else:  # If $virsh-iface list did not return any device, check virsh nodedev-list net:noh
-        net_devices_str = m.execute("virsh nodedev-list net")
-        net_devices_str = net_devices_str.split("\n", 2)[0]
-        device = net_devices_str.split("_", 2)[1]  # Ignore prefix (example: net_enp0s31f6_8c_16_45_5f_77_34)
-
-    return device
+def getNetworkDefaultRouteDevice(m):
+    routes = json.loads(m.execute("ip --json route show default"))
+    return routes[0]["dev"]
 
 
 @testlib.nondestructive
@@ -498,8 +491,8 @@ class TestMachinesNetworks(machineslib.VirtualMachinesCase):
             xfail_error="IPv4 address cannot be same as the network's broadcast address",
         ).execute()
 
-        # Test network devices
-        device = getNetworkDevice(m)
+        # Test network devices: create a new network NATing to a host iface
+        device = getNetworkDefaultRouteDevice(m)
         NetworkCreateDialog(
             self,
             name="test_network",