qapi-introspect: Mangle --prefix argument properly for C

qapi-introspect.py --prefix hasn't been used so far, but fix it anyway.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <1488544368-30622-7-git-send-email-armbru@redhat.com>
[Commit message improved]
Reviewed-by: NEric Blake <eblake@redhat.com>

scripts/qapi-introspect.py

@@ -64,7 +64,7 @@ class QAPISchemaGenIntrospectVisitor(QAPISchemaVisitor):
         # generate C
         # TODO can generate awfully long lines
         jsons.extend(self._jsons)
-        name = prefix + 'qmp_schema_json'
+        name = c_name(prefix, protect=False) + 'qmp_schema_json'
         self.decl = mcgen('''
 extern const char %(c_name)s[];
 ''',