Use StringIO instad of BytesIO in Py 3

The print function in Python will convert printed arguments to text
strings [1] and thus print() cannot be used with binary mode file
objects.

In Python 2 the write() method of BytesIO() takes as input *bytes* object
which refers to *str* in Python 3. [2] The write() method of StringIO()
takes *unicode* object. [3] Therefore, StringIO() object cannot be used with
the print() function.

In Python 3 the write() method of BytesIO() takes a *bytes* object (not
*str*). [4] Therefore, the BytesIO() obj cannot be used with print().
However, the write() method of StringIO() in Python 3 takes a *str* as
input.[5]

Example of the issue:

    from __future__ import print_function
    import io

    a = io.BytesIO()
    b = io.StringIO()

    print("test", file=a)  <- Fails for Python 3
    print("test", file=b)  <- Fails for Python 2

    a.write('%s\n' % "test")  <- Fails for Python 3
    b.write('%s\n' % "test")  <- Fails for Python 2

[1] https://docs.python.org/3.5/library/functions.html#print
[2] https://docs.python.org/2/library/io.html#io.BufferedWriter.write
[3] https://docs.python.org/2/library/io.html#io.TextIOBase.write
[4] https://docs.python.org/3/library/io.html#io.BufferedWriter.write
[5] https://docs.python.org/3/library/io.html#io.TextIOBase.write

tests/clitest.py

@@ -146,7 +146,11 @@ class Command(object):
         oldstdin = sys.stdin
         oldargv = sys.argv
         try:
-            out = io.BytesIO()
+            if sys.version_info[0] == 3:
+                out = io.StringIO()
+            else:
+                out = io.BytesIO()
+
             sys.stdout = out
             sys.stderr = out
             sys.argv = self.argv

tests/virtconvtest.py

@@ -20,6 +20,7 @@ from __future__ import print_function
 import glob
 import io
 import os
+import sys
 import unittest
 
 from virtconv import VirtConverter
@@ -32,7 +33,12 @@ out_dir = base_dir + "libvirt_output"
 
 class TestVirtConv(unittest.TestCase):
     def _convert_helper(self, infile, outfile, in_type, disk_format):
-        outbuf = io.BytesIO()
+
+        if sys.version_info[0] == 3:
+            outbuf = io.StringIO()
+        else:
+            outbuf = io.BytesIO()
+
         def print_cb(msg):
             print(msg, file=outbuf)