qcow2: Really use cache=unsafe for image creation

For cache=unsafe we also need to set BDRV_O_CACHE_WB, otherwise we have some
strange unsafe writethrough mode.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
(cherry picked from commit e1a7107f2d)

block/qcow2.c

@@ -975,7 +975,8 @@ static int qcow2_create2(const char *filename, int64_t total_size,
      */
     BlockDriver* drv = bdrv_find_format("qcow2");
     assert(drv != NULL);
-    ret = bdrv_open(bs, filename, BDRV_O_RDWR | BDRV_O_NO_FLUSH, drv);
+    ret = bdrv_open(bs, filename,
+        BDRV_O_RDWR | BDRV_O_CACHE_WB | BDRV_O_NO_FLUSH, drv);
     if (ret < 0) {
         goto out;
     }