qemu: Fix copy_paste_error in qemuBuildDriveStr.

Fix for this: http://www.redhat.com/archives/libvir-list/2014-November/msg00324.htmlSigned-off-by: NMatthias Gatto <matthias.gatto@outscale.com>

src/qemu/qemu_command.c

@@ -3762,7 +3762,7 @@ qemuBuildDriveStr(virConnectPtr conn,
                           disk->blkdeviotune.write_iops_sec_max);
     }
 
-    if (disk->blkdeviotune.write_iops_sec_max) {
+    if (disk->blkdeviotune.size_iops_sec) {
         virBufferAsprintf(&opt, ",iops_size=%llu",
                           disk->blkdeviotune.size_iops_sec);
     }