qapi: Fix cgen() for Python older than 2.7

A feature new in Python 2.7 crept into commit 77e703b8: re.subn()'s
fifth argument.  Avoid that, use re.compile().
Reported-by: NLaurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Tested-by: NLaurent Desnogues <laurent.desnogues@gmail.com>
Message-id: 1441640755-23902-1-git-send-email-armbru@redhat.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

scripts/qapi.py

@@ -944,7 +944,9 @@ def cgen(code, **kwds):
     raw = code % kwds
     if indent_level:
         indent = genindent(indent_level)
-        raw = re.subn("^.", indent + r'\g<0>', raw, 0, re.MULTILINE)
+        # re.subn() lacks flags support before Python 2.7, use re.compile()
+        raw = re.subn(re.compile("^.", re.MULTILINE),
+                      indent + r'\g<0>', raw)
         raw = raw[0]
     return re.sub(re.escape(eatspace) + ' *', '', raw)