kvm tools, qcow: Fix copy-on-write image corruption

We don't handle refcount table properly so make sure we only write to clusters
that have the "copied" flag set.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>