qemu|qtest: Avoid dangerous arguments

The list object is mutable in python and potentially might modify other
object's arguments when used as default argument. Reproducer:

    >>> vm1 = QEMUMachine("qemu")
    >>> vm2 = QEMUMachine("qemu")
    >>> vm1._wrapper.append("foo")
    >>> print vm2._wrapper
    ['foo']

In this case the `args` is actually copied so it would be safe to keep
it, but it's not a good practice to keep it. The same issue applies in
inherited qtest module.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-Id: <20170818142613.32394-3-ldoktor@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

scripts/qemu.py

@@ -30,7 +30,7 @@ class QEMUMachine(object):
         # vm is guaranteed to be shut down here
     '''
 
-    def __init__(self, binary, args=[], wrapper=[], name=None,
+    def __init__(self, binary, args=None, wrapper=None, name=None,
                  test_dir="/var/tmp", monitor_address=None,
                  socket_scm_helper=None, debug=False):
         '''
@@ -46,6 +46,10 @@ class QEMUMachine(object):
         @param debug: enable debug mode
         @note: Qemu process is not started until launch() is used.
         '''
+        if args is None:
+            args = []
+        if wrapper is None:
+            wrapper = []
         if name is None:
             name = "qemu-%d" % os.getpid()
         if monitor_address is None:

scripts/qtest.py

@@ -79,7 +79,7 @@ class QEMUQtestProtocol(object):
 class QEMUQtestMachine(qemu.QEMUMachine):
     '''A QEMU VM'''
 
-    def __init__(self, binary, args=[], name=None, test_dir="/var/tmp",
+    def __init__(self, binary, args=None, name=None, test_dir="/var/tmp",
                  socket_scm_helper=None):
         if name is None:
             name = "qemu-%d" % os.getpid()