qapi: Eliminate accidental global frontend state

The frontend can't be run more than once due to its global state.
A future commit will want to do that.

The only global frontend state remaining is accidental:
QAPISchemaParser.__init__()'s parameter previously_included=[].
Python evaluates the default once, at definition time.  Any
modifications to it are visible in subsequent calls.  Well-known
Python trap.  Change the default to None and replace it by the real
default in the function body.  Use the opportunity to convert
previously_included to a set.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20191018074345.24034-4-armbru@redhat.com>

scripts/qapi/common.py

@@ -391,8 +391,9 @@ def check(self):
 
 class QAPISchemaParser(object):
 
-    def __init__(self, fname, previously_included=[], incl_info=None):
-        previously_included.append(os.path.abspath(fname))
+    def __init__(self, fname, previously_included=None, incl_info=None):
+        previously_included = previously_included or set()
+        previously_included.add(os.path.abspath(fname))
 
         try:
             if sys.version_info[0] >= 3: