Split block_copy to separate file, to be cleanly shared with backup-top
filter driver in further commits.

It's a clean movement, the only change is drop "static" from interface
functions.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Message-id: 20190920142056.12778-8-vsementsov@virtuozzo.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>