qcow2.py: Add required padding for header extensions

The qcow2 specification requires that the header extension data be
padded to round up the extension size to the next multiple of 8 bytes.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1416935562-7760-3-git-send-email-kwolf@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/qcow2.py

@@ -7,6 +7,10 @@ import string
 class QcowHeaderExtension:
 
     def __init__(self, magic, length, data):
+        if length % 8 != 0:
+            padding = 8 - (length % 8)
+            data += "\0" * padding
+
         self.magic  = magic
         self.length = length
         self.data   = data