trace: update qemu-trace-stap to Python 3

qemu-trace-stap does not support Python 3 yet:

  $ scripts/qemu-trace-stap list path/to/qemu-system-x86_64
  Traceback (most recent call last):
    File "scripts/qemu-trace-stap", line 175, in <module>
      main()
    File "scripts/qemu-trace-stap", line 171, in main
      args.func(args)
    File "scripts/qemu-trace-stap", line 118, in cmd_list
      print_probes(args.verbose, "*")
    File "scripts/qemu-trace-stap", line 114, in print_probes
      if line.startswith(prefix):
  TypeError: startswith first arg must be bytes or a tuple of bytes, not str

Now that QEMU requires Python 3.5 or later we can switch to pure Python
3.  Use Popen()'s universal_newlines=True argument to treat stdout as
text instead of binary.

Fixes: 62dd1048c0bd ("trace: add ability to do simple printf logging via systemtap")
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1787395
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20200107112438.383958-1-stefanha@redhat.com
Message-Id: <20200107112438.383958-1-stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

scripts/qemu-trace-stap

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 # -*- python -*-
 #
 # Copyright (C) 2019 Red Hat, Inc
@@ -18,8 +18,6 @@
 # You should have received a copy of the GNU General Public License
 # along with this program; if not, see <http://www.gnu.org/licenses/>.
 
-from __future__ import print_function
-
 import argparse
 import copy
 import os.path
@@ -104,7 +102,9 @@ def cmd_list(args):
         if verbose:
             print("Listing probes with name '%s'" % script)
         proc = subprocess.Popen(["stap", "-l", script],
-                                stdout=subprocess.PIPE, env=tapset_env(tapsets))
+                                stdout=subprocess.PIPE,
+                                universal_newlines=True,
+                                env=tapset_env(tapsets))
         out, err = proc.communicate()
         if proc.returncode != 0:
             print("No probes found, are the tapsets installed in %s" % tapset_dir(args.binary))