qapi-event: Clean up how name of enum QAPIEvent is made

Use c_name() instead of ad hoc code.  Doesn't upcase the -p prefix,
which is an improvement in my book.  Unbreaks prefix containing '.',
but other funny characters remain broken.  To be fixed next.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
diff --git a/scripts/qapi-event.py b/scripts/qapi-event.py
index 56bc602..cc74f4d 100644
--- a/scripts/qapi-event.py
+++ b/scripts/qapi-event.py
@@ -267,7 +267,7 @@
 
 exprs = parse_schema(input_file)
 
-event_enum_name = prefix.upper().replace('-', '_') + "QAPIEvent"
+event_enum_name = c_name(prefix + "QAPIEvent", protect=False)
 event_enum_values = []
 event_enum_strings = []