qcow2: make more generic interface for qcow2_compress Give explicit size both for source and destination buffers, to make it similar with decompression path and than cleanly reuse parameter structure for decompression threads. Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> Reviewed-by: Alberto Garcia <berto@igalia.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>