qemu-iotests: Test unaligned sub-block zero write

Test zero write in byte range 512~1024 for 4k alignment.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1431522721-3266-4-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit ab53c447)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>