qcow2: Reduce is_zero() rounding

Now that bdrv_is_allocated accepts non-aligned inputs, we can
remove the TODO added in earlier refactoring.
Signed-off-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -2976,22 +2976,16 @@ static bool is_zero(BlockDriverState *bs, int64_t offset, int64_t bytes)
 {
     int64_t nr;
     int res;
-    int64_t start;
-
-    /* TODO: Widening to sector boundaries should only be needed as
-     * long as we can't query finer granularity. */
-    start = QEMU_ALIGN_DOWN(offset, BDRV_SECTOR_SIZE);
-    bytes = QEMU_ALIGN_UP(offset + bytes, BDRV_SECTOR_SIZE) - start;
 
     /* Clamp to image length, before checking status of underlying sectors */
-    if (start + bytes > bs->total_sectors * BDRV_SECTOR_SIZE) {
-        bytes = bs->total_sectors * BDRV_SECTOR_SIZE - start;
+    if (offset + bytes > bs->total_sectors * BDRV_SECTOR_SIZE) {
+        bytes = bs->total_sectors * BDRV_SECTOR_SIZE - offset;
     }
 
     if (!bytes) {
         return true;
     }
-    res = bdrv_block_status_above(bs, NULL, start, bytes, &nr, NULL, NULL);
+    res = bdrv_block_status_above(bs, NULL, offset, bytes, &nr, NULL, NULL);
     return res >= 0 && (res & BDRV_BLOCK_ZERO) && nr == bytes;
 }