From http://patchwork.ozlabs.org/patch/137186/

diff -ru qemu-kvm-0.15.1-orig/block/vdi.c qemu-kvm-0.15.1/block/vdi.c
--- qemu-kvm-0.15.1-orig/block/vdi.c	2011-10-19 15:54:48.000000000 +0200
+++ qemu-kvm-0.15.1/block/vdi.c	2012-02-25 18:15:36.114574574 +0100
@@ -1,7 +1,7 @@
 /*
  * Block driver for the Virtual Disk Image (VDI) format
  *
- * Copyright (c) 2009 Stefan Weil
+ * Copyright (c) 2009, 2012 Stefan Weil
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -767,8 +767,12 @@
             acb->header_modified = 1;
         }
         acb->bmap_last = block_index;
+        /* Copy data to be written to new block and zero unused parts. */
+        memset(block, 0, sector_in_block * SECTOR_SIZE);
         memcpy(block + sector_in_block * SECTOR_SIZE,
                acb->buf, n_sectors * SECTOR_SIZE);
+        memset(block + (sector_in_block + n_sectors) * SECTOR_SIZE, 0,
+               (s->block_sectors - n_sectors - sector_in_block) * SECTOR_SIZE);
         acb->hd_iov.iov_base = (void *)block;
         acb->hd_iov.iov_len = s->block_size;
         qemu_iovec_init_external(&acb->hd_qiov, &acb->hd_iov, 1);