block: introduce compress filter driver

block: introduce compress filter driver

Allow writing all the data compressed through the filter driver.
The written data will be aligned by the cluster size.
Based on the QEMU current implementation, that data can be written to
unallocated clusters only. May be used for a backup job.

Suggested-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 1575288906-551879-2-git-send-email-andrey.shinkevich@virtuozzo.com
[mreitz: Replace NULL bdrv_get_format_name() by "(no format)"]
Signed-off-by: Max Reitz <mreitz@redhat.com>