qemu-img: Convert by cluster size if target is compressed

If target block driver forces compression, qemu-img convert needs to
write by cluster size as well as "-c" option.

Particularly, this applies for converting to VMDK streamOptimized
format.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

include/block/block.h

@@ -34,6 +34,10 @@ typedef struct BlockDriverInfo {
      * opened with BDRV_O_UNMAP flag for this to work.
      */
     bool can_write_zeroes_with_unmap;
+    /*
+     * True if this block driver only supports compressed writes
+     */
+    bool needs_compressed_writes;
 } BlockDriverInfo;
 
 typedef struct BlockFragInfo {

qemu-img.c

@@ -1502,6 +1502,7 @@ static int img_convert(int argc, char **argv)
             goto out;
         }
     } else {
+        compress = compress || bdi.needs_compressed_writes;
         cluster_sectors = bdi.cluster_size / BDRV_SECTOR_SIZE;
     }