image-fuzzer: Write bytes instead of string to image file

This is necessary for Python 3 compatibility.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20191016192430.25098-3-ehabkost@redhat.com
Message-Id: <20191016192430.25098-3-ehabkost@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

tests/image-fuzzer/qcow2/layout.py

@@ -518,7 +518,7 @@ class Image(object):
         rounded = (size + self.cluster_size - 1) & ~(self.cluster_size - 1)
         if rounded > size:
             image_file.seek(rounded - 1)
-            image_file.write("\0")
+            image_file.write(b'\x00')
         image_file.close()
 
     @staticmethod