iotests: 147: Don't test inet6 if not available

This is the case in our docker tests, as we use --net=none there. Skip
this method.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/147

@@ -147,6 +147,13 @@ class BuiltinNBD(NBDBlockdevAddBase):
         self._server_down()
 
     def test_inet6(self):
+        try:
+            socket.getaddrinfo("::0", "0", socket.AF_INET6,
+                               socket.SOCK_STREAM, socket.IPPROTO_TCP,
+                               socket.AI_ADDRCONFIG | socket.AI_CANONNAME)
+        except socket.gaierror:
+            # IPv6 not available, skip
+            return
         address = { 'type': 'inet',
                     'data': {
                         'host': '::1',