qapi: Simplify guardname()

The guards around built-in declarations lose their _H.  It never made
much sense anyway.

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

scripts/qapi.py

@@ -958,14 +958,9 @@ def mcgen(code, **kwds):
         code = code[1:]
     return cgen(code, **kwds)
 
-def basename(filename):
-    return filename.split("/")[-1]
 
 def guardname(filename):
-    guard = basename(filename).rsplit(".", 1)[0]
-    for substr in [".", " ", "-"]:
-        guard = guard.replace(substr, "_")
-    return guard.upper() + '_H'
+    return c_name(filename, protect=False).upper()
 
 def guardstart(name):
     return mcgen('''
@@ -1035,6 +1030,7 @@ def parse_command_line(extra_options = "", extra_long_options = []):
 
 def open_output(output_dir, do_c, do_h, prefix, c_file, h_file,
                 c_comment, h_comment):
+    guard = guardname(prefix + h_file)
     c_file = output_dir + prefix + c_file
     h_file = output_dir + prefix + h_file
 
@@ -1067,7 +1063,7 @@ def maybe_open(really, name, opt):
 #define %(guard)s
 
 ''',
-                      comment = h_comment, guard = guardname(h_file)))
+                      comment = h_comment, guard = guard))
 
     return (fdef, fdecl)