qcow2.py: Add required padding for header extensions

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>
(cherry picked from commit 8884dd1bbc5ce42cd657ffcbef3a477443468974)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>