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: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>