python/machine.py: change default wait timeout to 3 seconds

Machine.wait() does not appear to be used except in the acceptance tests,
and an infinite timeout by default in a test suite is not the most helpful.

Change it to 3 seconds, like the default shutdown timeout.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20200710050649.32434-13-jsnow@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

python/qemu/machine.py

@@ -486,12 +486,12 @@ def kill(self):
         """
         self.shutdown(hard=True)
 
-    def wait(self, timeout: Optional[int] = None) -> None:
+    def wait(self, timeout: Optional[int] = 3) -> None:
         """
         Wait for the VM to power off and perform post-shutdown cleanup.
 
         :param timeout: Optional timeout in seconds.
-                        Default None, an infinite wait.
+                        Default 3 seconds, A value of None is an infinite wait.
         """
         self.shutdown(has_quit=True, timeout=timeout)