stage3: Fetcher.fetch_stage: Pass stderr fd to wget

The main entry point of the `mkvm` program closes FD 3 (native stderr)
to prevent extraneous messages from libvirt. Unfortunately, this breaks
wget's progress indicator, as it is printed to stderr as well. To ensure
that the progress indicator works, we have to pass the FD of the copy of
stderr to wget.

src/mkvm/stage3.py

@@ -6,6 +6,7 @@ import io
 import os
 import shutil
 import subprocess
+import sys
 import time
 import urllib.parse
 import urllib.request
@@ -102,7 +103,7 @@ class Fetcher(object):
         cmd += uris
         log.debug('Running command: {}'.format(' '.join(cmd)))
         try:
-            p = subprocess.Popen(cmd, cwd=self.cache_dir)
+            p = subprocess.Popen(cmd, cwd=self.cache_dir, stderr=sys.stderr)
         except OSError as e:
             raise FetchError('Failed to run wget: {}'.format(e))
         if p.wait() != 0: