instead of filling them with zeroes
this fixes an issue where we could not restore a container with large
sparse files in the backup (e.g. a 10GiB sparse file in a container
with a 8GiB disk)
if the last operation of the copy was a seek, we need to truncate
the file to the correct size (seek beyond filesize does not change it)
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>