net: raise an error if -net type is invalid

When a -net type is used that was not compiled into the binary there
should be an error message.

Note the special case for -net none, which is a no-op.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Message-id: 1432743412-15943-4-git-send-email-stefanha@redhat.com

net/net.c

@@ -942,6 +942,17 @@ static int net_client_init1(const void *object, int is_netdev, Error **errp)
         }
         /* missing optional values have been initialized to "all bits zero" */
         name = u.net->has_id ? u.net->id : u.net->name;
+
+        if (opts->kind == NET_CLIENT_OPTIONS_KIND_NONE) {
+            return 0; /* nothing to do */
+        }
+
+        if (!net_client_init_fun[opts->kind]) {
+            error_setg(errp, QERR_INVALID_PARAMETER_VALUE, "type",
+                       "a net backend type (maybe it is not compiled "
+                       "into this binary)");
+            return -1;
+        }
     }
 
     if (net_client_init_fun[opts->kind]) {