blockdev: do not default cache.no-flush to true

That's why all my VMs were so fast lately. :)

This changed in 1.6.0 by mistake in patch 29c4e2b5 (blockdev: Split up
'cache' option, 2013-07-18).

Cc: qemu-stable@nongnu.org
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

blockdev.c

@@ -443,7 +443,7 @@ static DriveInfo *blockdev_init(QemuOpts *all_opts,
     if (qemu_opt_get_bool(opts, "cache.direct", false)) {
         bdrv_flags |= BDRV_O_NOCACHE;
     }
-    if (qemu_opt_get_bool(opts, "cache.no-flush", true)) {
+    if (qemu_opt_get_bool(opts, "cache.no-flush", false)) {
         bdrv_flags |= BDRV_O_NO_FLUSH;
     }