T6764: Ethtool not all NICs can show ring-buffers

Not all NICs could provide ring-buffers info requested by ethtool
in JSON format
For example 'vif' Xen/XCP-NG interfaces
Fix it

python/vyos/ethtool.py

@@ -101,8 +101,9 @@ def __init__(self, ifname):
         self._features = loads(out)[0]
 
         # Get information about NIC ring buffers
-        out, _ = popen(f'ethtool --json --show-ring {ifname}')
-        self._ring_buffer = loads(out)[0]
+        out, err = popen(f'ethtool --json --show-ring {ifname}')
+        if not bool(err):
+            self._ring_buffer = loads(out)[0]
 
         # Get current flow control settings, but this is not supported by
         # all NICs (e.g. vmxnet3 does not support is)