ethtool: T3871: honor the unsupported-driver denylist for flow-control too

Flow-control commits failed intermittently on virtio_net and similar
paravirtualized drivers: they support reading pause parameters but not
setting them, and that mismatch varies across kernel/QEMU versions.

The existing denylist for this already covered speed/duplex changes and its
name implied flow-control too, but nothing actually consulted it there - the
check relied solely on a live probe. The paired smoketest branched on that same
raw probe instead of the fixed capability check, so it missed the same drivers.

Assisted-by: Claude:claude-sonnet-5

Author: c-po

python/vyos/ethtool.py

@@ -217,6 +217,8 @@ def check_speed_duplex(self, speed, duplex):
 
     def check_flow_control(self):
         """ Check if the NIC supports flow-control """
+        if self.get_driver_name() in _drivers_without_speed_duplex_flow:
+            return False
         return bool(self._flow_control)
 
     def get_flow_control(self):

smoketest/scripts/cli/test_interfaces_ethernet.py

@@ -287,17 +287,21 @@ def test_ethtool_coalesce(self):
 
     def test_ethtool_flow_control(self):
         for interface in self._interfaces:
+            ethtool = Ethtool(interface)
             # Disable flow-control
             self.cli_set(self._base_path + [interface, 'disable-flow-control'])
-            # Check current flow-control state on ethernet interface
-            out, err = popen(f'sudo ethtool --json --show-pause {interface}')
-            # Flow-control not supported - test if it bails out with a proper
-            # this is a dynamic path where err = 1 on VMware, but err = 0 on
-            # a physical box.
-            if bool(err):
+
+            # Ask the same capability check the CLI commit itself uses,
+            # rather than a raw ethtool --show-pause probe: some drivers
+            # (virtio_net, vmxnet3, xen_netfront, ...) support querying
+            # pause parameters but not changing them, so a bare --show-pause
+            # exit code is not a reliable predictor of whether the commit
+            # will succeed.
+            if not ethtool.check_flow_control():
                 with self.assertRaises(ConfigSessionError):
                     self.cli_commit()
             else:
+                out, err = popen(f'sudo ethtool --json --show-pause {interface}')
                 out = loads(out)
                 # Flow control is on
                 self.assertTrue(out[0]['autonegotiate'])