Python: Use correct ptid in btrace recording

The user would always get the instruction_history and function_call_history
objects of the current thread, not the thread for which the gdb.Record object
was created.

The attached testcase fails without this patch and passes with the patch.

gdb/python/py-record.h

@@ -0,0 +1,40 @@
+/* Python interface to record targets.
+
+   Copyright 2017 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef GDB_PY_RECORD_H
+#define GDB_PY_RECORD_H
+
+#include "inferior.h"
+#include "python-internal.h"
+#include "record.h"
+
+/* Python Record object.  */
+
+typedef struct
+{
+  PyObject_HEAD
+
+  /* The ptid this object refers to.  */
+  ptid_t ptid;
+
+  /* The current recording method.  */
+  enum record_method method;
+} recpy_record_object;
+
+#endif /* GDB_PY_RECORD_H */