iotests: add test for unaligned granularity bitmap backup

iotests: add test for unaligned granularity bitmap backup

Start a VM with a 4097 byte image attached, add a 4096 byte granularity
dirty bitmap, mark it dirty, and then do a backup.

This used to run into an assert and fail, check that it works as
expected and also check the created image to ensure that misaligned
backups in general work correctly.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
Message-Id: <20200810095523.15071-2-s.reiter@proxmox.com>
[mreitz: Drop bitmap, and do not write past the image's end]
Signed-off-by: Max Reitz <mreitz@redhat.com>